errors in submitting data into database in php laravel

I'm trying to submit data into the database using laravel. I got an error Controller does not exist when it clearly exist. Here is my code

 RequestInsertController.php
<?php
namespace App\Http\Controllers;
use Illuminate\Http\Request;
use DB;
use App\Http\Controllers\Controller;
use App\Http\Requests;
class RequestInsertController extends Controller
{
// the following function is responsible for rendering the form view
public function insform(){
return view('requestform');
}
public function postdata(Request $req){
// handeling first serction of multi step form 
$firstname = $req ->input('firstName'); // taking data from the firstname text field on the request form 
$lastname  = $req ->input('lastName'); // taking data form the lastname text field on the request form
$phone_num = $req ->input('phone_number'); // ||
$address   = $req ->input('address'); // ||
$numAdults = $req ->input('numberofAdults'); //||
$numkids   = $req ->input('numberofChildren');
$data = array('headofhousehold_firstname' => $firstname, "headofhousehold_lastname" => $lastname, "phone" => $phone_num, "address" => $address, "numofadults" => $numAdults, "numofkids" => $numkids);
DB::table('family') -> insert($data);
echo"record successfuly inserted";
return redirect()->back()->with('success', 'Data inserted successfully.');
}
}

here is my web.php

       <?php
use Illuminate\Support\Facades\Route;
use App\Http\Controllers\AuthManager;
use App\Http\Controllers\orderitemController;
use App\Http\Controllers\LoginController;
use App\Http\Controllers\RequestInsertController;
//Route::get('postdata','RequestInsertController@insform'); // this route is used to display 
//the form view when /postdata is accessed 
Route::post('requestFormData','RequestInsertController@postdata');   
/* Route::get('requestform', [RequestInsertController::class, 'postdata']);
Route::post('requestform', [RequestInsertController::class, 'insform']); */
Route::get('requestform', [RequestInsertController::class, 'insform'])->name('request.form');
Route::post('requestform', [RequestInsertController::class, 'postdata'])- 
>name('request.form.submit');

and in my requestform.blade.php

    <form action="{{url('requestFormData')}}" id="multi-step-form" class="multi-step-form" method="POST">
<div>
@csrf
<section class ="form-section">
<h4>Client Info</h4>
<div class="row">
<div class="col-lg-2 mb-4">
<div class="form-group">
<label class="text-label">First Name*</label>
<input type="text" name="firstName" class="form-control" placeholder="FirstName" required>
</div>
</div>
<div class="col-lg-2 mb-4">
<div class="form-group">
<label class="text-label">Last Name*</label>
<input type="text" name="lastName" class="form-control" placeholder="LastName" required>
</div>
</div>
<div class="col-lg-5 mb-4">
<div class="form-group">
<label class="text-label">Phone Number*</label>
<input type="text" name="phone number" class="form-control" placeholder="Phone Number" required>
</div>
</div>
<div class="col-lg-5 mb-4">
<div class="form-group">
<label class="text-label">Move in date*</label>
<input type="text" name="moveindate" class="form-control" placeholder="Move In Date" required>
</div>
</div>
<div class="col-lg-12 mb-4">
<div class="form-group">
<label class="text-label"> Address</label>
<div class="input-group">
<input type="text" class="form-control" id="inputGroupPrepend2" aria-describedby="inputGroupPrepend2" placeholder="address 1" required>
</div>
</div>
</div>
</div>
</div>
</section>
<div class = "form-navigation mt-3">
<button type="button" class ="  previous btn btn-primary float-left"> previous </button>
<button type="button" class =" next btn btn-primary float-right">Next  </button>
<button type="submit" class =" btn btn-success float-right"> Submit </button>
</div>
</form>

please guide me into submitting data into the database. I don't know what i am doing wrong. I get the following error
>The RequestInsertController does not exist

Your error is in this line:

Route::post('requestFormData','RequestInsertController@postdata');  

The namespace is not defined.

Replace with :

Route::post('requestFormData',[RequestInsertController::class, 'postdata'])->name('postdata');

This line will work because the RequestInsertController is defined at the top of your route with :

use App\Http\Controllers\RequestInsertController;