laravel throwing MethodNotAllowedHttpException

You are getting that error because you are posting to a GET route.

I would split your routing for validate into a separate GET and POST routes.

New Routes:

Route::post('validate', 'MemberController@validateCredentials');

Route::get('validate', function () {
    return View::make('members/login');
});

Then your controller method could just be

public function validateCredentials()
{
    $email = Input::post('email');
    $password = Input::post('password');
    return "Email: " . $email . " and Password: " . $password;
}

My suspicion is the problem lies in your route definition.

You defined the route as a GET request but the form is probably sending a POST request. Change your route definition to match the form's request method.

Route::post('/validate', [MemberController::class, 'validateCredentials']);

It's generally better practice to use named routes (helps to scale if the controller method/class changes).

Route::post('/validate', [MemberController::class, 'validateCredentials'])
    ->name('member.validateCredentials');

In the view, use the validation route as the form's action.

<form action="{{ route('member.validateCredentials') }}" method="POST">
  @csrf
...
</form>

The problem is the you are using POST but actually you have to perform PATCH To fix this add

<input name="_method" type="hidden" value="PATCH">

Just after the Form::model line

That is because you are posting data through a get method.

Instead of

Route::get('/validate', 'MemberController@validateCredentials');

Try this

Route::post('/validate', 'MemberController@validateCredentials');