the username validation should only be done on the server side because only the server has the needed information. in this case, you can use ajax-based validation.

step 1 − to enable the ajax validation, modify the registration view this way.

<?php
   use yii\bootstrap\activeform;
   use yii\bootstrap\html;
?>
  
<div class = "row">
   <div class = "col-lg-5">  
   
      <?php $form = activeform::begin(['id' => 'registration-form', 
         'enableajaxvalidation' => true]); ?>  
      <?= $form->field($model, 'username') ?>  
      <?= $form->field($model, 'password')->passwordinput() ?>  
      <?= $form->field($model, 'email')->input('email') ?>  
      <?= $form->field($model, 'country') ?>  
      <?= $form->field($model, 'city') ?>  
      <?= $form->field($model, 'phone') ?>  
      <div class = "form-group">             
         <?= html::submitbutton('submit', ['class' => 'btn btn-primary', 
            'name' => 'registration-button']) ?>
      </div>
      
      <?php activeform::end(); ?>  
   </div>
</div>

we should also prepare the server, so that it can handle the ajax requests.

step 2 − modify the actionregistration method of the sitecontroller this way.

public function actionregistration() { 
   $model = new registrationform(); 
   if (yii::$app->request->isajax && $model->load(yii::$app->request>post())) { 
      yii::$app->response->format = response::format_json; 
      return activeform::validate($model); 
   } 
   return $this->render('registration', ['model' => $model]); 
}

step 3 − now, go to http://localhost:8080/index.php?r=site/registration, you will notice that the form validation is done by ajax requests.