Function Auth does not work for me in Laravel php

I did a manual authentication in Laravel 5.4 and I can not get it to return true the function.

In my database, enter a record from the database administrator with username = admin and password = admin. (I also tried to create the username and password from a laravel form and nothing)

here I leave the code of my files

LoginController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Auth;

class LoginController extends Controller
{

public function store( Request $request ){


    if(Auth::attempt(['username' => 'admin', 'password' => 'admin'])){
        return "ok";
    }else{
        return "error";
    }
}
}

Obviously the username and password are hard-coded here so that you do not have to enter them again and again.

User.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model as Eloquent;
use Illuminate\Contracts\Auth\Authenticatable;
use Illuminate\Auth\Authenticatable as AuthenticableTrait;

class Usuario extends Eloquent implements Authenticatable
{
use AuthenticableTrait;

protected $fillable = ['username', 'password'];

}

login form

{!!Form::open(['route' => 'login.store' ,'method' => 'POST'])!!}
    <div class="form-group">
        {!!Form::label('Usuario')!!}
        {!!Form::text('user', null, ['class' => 'form-control'])!!}
    </div>        
    <div class="form-group">
        {!!Form::label('Contraseña')!!}
        {!!Form::password('password', ['class' => 'form-control'])!!}
    </div>
    <div class="form-group">
        {!!Form::submit('Ingresar', ['class' => 'btn btn-primary btn-lg 
btn-block'])!!}
    </div>
    {!!Form::close()!!}

I know that the function arrives, since it is giving me "error" as it can be seen in the logincontroller, in the part of the else.

I think that it is not correctly comparing the password since I did a test to change the name of the column 'username' in the database and I throw an error, but when changing the name of the column 'password' only he returned false and no error of Laravel.