Auth::attempt($credential)

Method Illuminate\Auth\RequestGuard::attempt does not exist

public static function loginUser($credentials) {
   if (Auth::attempt($credentials)) {
       echo "<pre>";
       print_r('ok');
       exit;
   }
   return [];
}

<?php

namespace App;

use IlluminateSupportFacadesAuth;
use IlluminateAuthAuthenticatable;
use LaravelLumenAuthAuthorizable;
use IlluminateDatabaseEloquentModel;
use IlluminateContractsAuthAuthenticatable as AuthenticatableContract;
use IlluminateContractsAuthAccessAuthorizable as AuthorizableContract;

class User extends BaseModel implements AuthenticatableContract, AuthorizableContract {

    use Authenticatable,
        Authorizable;

    /**
     * modal table
     * @var type
     */
    protected $table = 'users';

    /**
     * modal primary key
     * @var type
     */
    protected $primarykey = 'id';

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = ['full_name', 'username',
           'email','password',  'picture'
    ];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [];


    public static function saveNewUser($inputs) {
        $user = new User($inputs);
        if ($user->save()) {
            return $user;
        }
        return [];
    }


    /**
     * login user
     * @param type $credentials
     * @return type
     */
    public static function loginUser($credentials) {
        if (Auth::attempt($credentials)) {
            echo "<pre>";
            print_r('ok');
            exit;
        }
        return [];
    }

}