Why can't I call an interface with a collection of methods from the main package

I am really new to golang and I am trying to see how encapsulation really works in go.

I have the following structure

-- package a
    -a_core.go
    -a.go
    -models.go

-- main.go

In models.go I have structs for request and responses for an api call,

a.go has an empty struct, which is private and a public interface, which I want to expose with various methods

a_core.go just has some business logic which would be called in my interface implementation

Then, I have a main.go where I just call the public interface.

code in a.go

package a

type myFunction struct{}

type MyFunc interface {
 Create(myData *MyData) (*MyData, error)
 Fetch(test string)
 Delete(test string)
}

//Concrete implementations that can be accessed publicly
func (a *myFunction) Create(data *MyData) (*MyData, error) {
  return nil, nil	
}

func (a *myFunction) Fetch(test string) {

}

func (a *myFunction) Delete(test string) {

}

In main.go, I call the interface my first create the MyData pointer with values

data := &a.MyData{
 /////
}

result, err := a.MyFunc.Create(data)

I get the following error when I do this,

too few arguments in call to a.MyFunc.Create

cannot use data (variable of type *a.MyData) as a.MyFunc value in argument to a.MyFunc.Create: missing method CreatecompilerInvalidIfaceAssign

Please what am I doing wrong?

Here is an example
Note that names in uppercase are public, in lowercase private (see https://tour.golang.org/basics/3 )

./go-example/main.go

package main

import "go-example/animal"

func main() {
	var a animal.Animal
	a = animal.Lion{Age: 10}
	a.Breathe()
	a.Walk()
}

./go-example/animal/animal.go

package animal

import "fmt"

type Animal interface {
	Breathe()
	Walk()
}

type Lion struct {
	Age int
}

func (l Lion) Breathe() {
	fmt.Println("Lion breathes")
}

func (l Lion) Walk() {
	fmt.Println("Lion walk")
}