Yii2 ajax validation not work

I was trying to validate my YII2 register form but it not work. In view:

$form = ActiveForm::begin([
                    'id'=>'register',
                    'options'=>[
                        'accept-charset'=>'utf-8'
                    ],
                    'validateOnChange'=> false,
                    'enableAjaxValidation' => true,
                    'validateOnSubmit'=>true,
                ])

In controller:

$model = new MUser();
        if($model->load(Yii::$app->request->post()) && Yii::$app->request->isAjax)
        {
            $model->refresh();
            Yii::$app->response->format = 'json';
            return ActiveForm::validate($model);
        }

In Model:

public function rules()
    {
        return [
    ['username', 'unique', 'targetClass'=>'com\modules\admin\models\MUser',
                'message'=>'Username exist')]
     ];

    }

Can anyone let me know what wrong I am doing? Plz