yii\base\Model::addError PHP Method

    public function addError($attribute, $error = '')
    {
        $this->_errors[$attribute][] = $error;
    }

 /**
  * server validatiom for unique zip_code
  * @param \yii\base\Model $model
  * @param string $attribute
  */
 public function validateAttribute($model, $attribute)
 {
     $value = $model->{$attribute};
     if (!Locations::find()->where(['zip_code' => $value])->exists()) {
         $model->addError($attribute, $this->message);
     }
 }