Fragments on my app are not appearing after button click

I'm new to the android development game and I have researched and read tons of info on fragments, but I seem to have run into a bit of a snag. After creating everything and checking to make sure I had zero errors my app isn't working the way I'd like. On button click I would like for a fragment to show up on the screen. I have two buttons, so two fragments, however my buttons don't seem to work.

Heres my layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="35dp">

    <RelativeLayout
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1">

    <Button
        android:id="@+id/signin_button"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:keepScreenOn="true"
        android:gravity="center"
        android:text="Sign In"
        android:textColor="#000000"
        android:textSize="18sp"
        android:textStyle="bold" />

    </RelativeLayout>
    <RelativeLayout
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1">

    <Button
        android:id="@+id/signup_button"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:keepScreenOn="true"
        android:gravity="center"
        android:text="Sign Up"
        android:textColor="#000000"
        android:textSize="18sp"
        android:textStyle="bold" />

    </RelativeLayout>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="10dp"
        android:id="@+id/parent_layout"
        >

        <FrameLayout
        android:id="@+id/signin_fragment"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_weight="1" />


    </LinearLayout>
    </LinearLayout>

Heres The java code:

package com.example.projectconviencesplash;

import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

public class SigninMainActivity extends Fragment implements View.OnClickListener {

    public SigninMainActivity() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.signinfragment, container, false);

        Button signinBtn = (Button) rootView.findViewById(R.id.signin_button);
        Button signupBtn = (Button) rootView.findViewById(R.id.signup_button);

        signinBtn.setOnClickListener(this);
        signupBtn.setOnClickListener(this);


        return rootView;
    }

    @Override
    public void onClick(View view) {
        Fragment fragment = null;
        switch (view.getId()) {
            case R.id.signin_button:
                fragment = new SigninTwoActivity();
                replaceFragment(fragment);
                break;

            case R.id.signup_button:
                fragment = new SignupTwoActivity();
                replaceFragment(fragment);
                break;
        }
    }

    public void replaceFragment(Fragment someFragment) {
        assert getFragmentManager() != null;
        FragmentTransaction transaction = getFragmentManager().beginTransaction();
        transaction.replace(R.id.signin_fragment, someFragment);
        transaction.addToBackStack(null);
        transaction.commit();
    }


}

Here's the fragment xml:

<?xml version="1.0" encoding="utf-8"?>

    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="10dp">

        <Button
            android:id="@+id/dummy_button"
            android:layout_width="match_parent"
            android:layout_height="35dp"
            android:layout_marginTop="20dp"
            android:layout_marginBottom="0dp"
            android:layout_weight="0"
            android:background="@drawable/fb_button"
            android:drawableStart="@drawable/fb"
            android:drawablePadding="-70dp"
            android:paddingStart="70dp"
            android:text="@string/fb_Title"
            android:textColor="#FFFFFF"
            android:textSize="18sp" />

        <Button
            android:id="@+id/dummy_button"
            style="@style/Widget.AppCompat.Button.Borderless"
            android:layout_width="match_parent"
            android:layout_height="35dp"
            android:layout_marginTop="10dp"
            android:layout_marginBottom="0dp"
            android:layout_weight="0"
            android:background="@drawable/g_button"
            android:drawableStart="@drawable/g"
            android:drawablePadding="-80dp"
            android:paddingStart="80dp"
            android:text="@string/g_Title"
            android:textColor="#FFFFFF"
            android:textSize="18sp" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="70dp">

            <ImageView
                android:layout_width="0dp"
                android:layout_height="match_parent"
                android:layout_weight="2"
                android:layout_marginStart="0dp"
                android:layout_marginLeft="0dp"
                android:layout_marginEnd="0dp"
                android:layout_marginRight="0dp"
                android:src="@drawable/strike" />

            <TextView
                android:layout_width="149dp"
                android:layout_height="match_parent"
                android:layout_weight="1"
                android:gravity="center"
                android:layout_marginStart="0dp"
                android:layout_marginLeft="0dp"
                android:layout_marginEnd="0dp"
                android:layout_marginRight="0dp"
                android:text="@string/email_Title"
                android:textColor="#cccccc"
                android:textSize="16sp"
                android:textStyle="" />

            <ImageView
                android:layout_width="0dp"
                android:layout_height="match_parent"
                android:layout_weight="2"
                android:layout_marginStart="0dp"
                android:layout_marginLeft="0dp"
                android:layout_marginEnd="0dp"
                android:layout_marginRight="0dp"
                android:src="@drawable/strike" />

        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="85dp"
            android:orientation="vertical">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginTop="20dp"
                android:layout_weight="1"
                android:gravity="start"
                android:text="@string/email"
                android:textColor="#000000"
                android:textSize="18sp"
                android:textStyle="" />

            <EditText
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginTop="10dp"
                android:layout_weight="1.5"
                android:background="@drawable/edittextbackground"
                android:inputType="text" />
        </LinearLayout>
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="85dp"
            android:orientation="vertical">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginTop="20dp"
                android:layout_weight="1"
                android:gravity="start"
                android:text="@string/password"
                android:textColor="#000000"
                android:textSize="18sp"
                android:textStyle="" />

            <EditText
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginTop="10dp"
                android:layout_weight="1.5"
                android:background="@drawable/edittextbackground"
                android:inputType="text"/>
        </LinearLayout>

        <Button
            android:id="@+id/dummy_button"
            style="@style/Widget.AppCompat.Button.Borderless"
            android:layout_width="match_parent"
            android:layout_height="35dp"
            android:layout_marginTop="20dp"
            android:layout_marginBottom="0dp"
            android:layout_weight="0"
            android:background="@drawable/rounded_corners"
            android:drawablePadding="-10dp"
            android:paddingStart="30dp"
            android:text="@string/signin"
            android:textColor="#FFFFFF"
            android:textSize="18sp" />

    </LinearLayout>

Lastly, here is the fragment java:

package com.example.projectconviencesplash;

import androidx.fragment.app.Fragment;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.app.FragmentManager;

public class SigninTwoActivity extends Fragment {
    @Override
    public View  onCreateView(LayoutInflater inflater, ViewGroup container,
                                     Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.signinfragment, container, false);
    }
}

Again, when I click on these buttons they don't seem to do anything. Also, if anyone is feeling particularly generous, I'd love to know how to make one of my fragments be default when loading this activity so that its not just a blank screen with two buttons when first loading. I appreciate all the help on this guys!

Use getChildFragmentManager instead of getFragmentManager when you want replace fragment inside a fragment.

I'm going to use a more basic code as it will be easier to follow along and hopefully help when it comes to implementing it in other projects. The code I provided below will have a MainActivity, FragmentOne, FragmentTwo, FragmentThree. The MainActivity will load FragmentOne on startup, FragmentOne will have two buttons one to load FragmentTwo and one to load FragmentThree.

activity_main.xml
This FrameLayout will hold the fragments, note the ID fragment_container.

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />

MainActivity
The activity should extend AppCompatActivity

public class MainActivity extends AppCompatActivity {
/**
* Called when the activity is created
*/
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Reference to Fragment one object
FragmentOne fragOne = new FragmentOne();
// Reference to fragment transaction.
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
// Fragment Transaction
fragmentTransaction.add(R.id.fragment_container, fragOne)
.addToBackStack(null) // Adds fragment to back stack, for backward navigation.
.commit(); // Finishes the fragment transaction.
}
}

fragment_one.xml
Nothing special here.

<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.LinearLayoutCompat xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".FragmentOne"
android:orientation="vertical">
<TextView
android:id="@+id/tv_title"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Fragment One"
android:textSize="32sp"
tools:layout_editor_absoluteX="143dp" />
<Button
android:id="@+id/btn_fragment_two"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Fragment two"
tools:layout_editor_absoluteY="401dp" />
<Button
android:id="@+id/btn_fragment_three"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Fragment three"
tools:layout_editor_absoluteY="401dp" />
</androidx.appcompat.widget.LinearLayoutCompat>

FragmentOne

public class FragmentOne extends Fragment {
/**
* Called when the fragment loads its user interface
*/
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_one,container, false);
// Reference the fragments
final FragmentTwo fragTwo = new FragmentTwo();
final FragmentThree fragThree = new FragmentThree();
// Create FragmentTransaction, getActivity returns context of the activity in a fragment.
final FragmentTransaction transaction = getActivity().getSupportFragmentManager().beginTransaction();
// Reference Button view.
Button loadFragTwoBtn = v.findViewById(R.id.btn_fragment_two);
// Set clickListener
loadFragTwoBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
transaction.replace(R.id.fragment_container, fragTwo)
.addToBackStack(null)
.commit();
}
});
Button loadFragThreeBtn = v.findViewById(R.id.btn_fragment_three);
loadFragThreeBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
transaction.replace(R.id.fragment_container, fragThree)
.addToBackStack(null)
.commit();
}
});
return v;
}
}

fragment_two.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".FragmentTwo">
<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="Fragment Two"
android:textSize="32sp"/>
</FrameLayout>

FragmentTwo

public class FragmentTwo extends Fragment {
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_two, container, false);
return view;
}
}

fragment_three
public class FragmentTwo extends Fragment {

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_two, container, false);
return view;
}

}

FragmentThree

public class FragmentThree extends Fragment {

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_three, container, false);
return view;
}

}

Hope this is helpful, I would also strongly recommend doing the fragment code labs to help build a strong foundation. 
Additional note, 
When providing an ID for a button the ID has to be unique, in your layout, all of your buttons use the same ID. 
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