Form validation throws a strict warning
There are contexts where Drupal allows you to use a notation like  '\Drupal\module_name\Controller\MyController::_field_p_rpservicename_validate' to refer to a public function of an object. In the case of validation handlers, since Drupal is calling them with call_user_func_array(), with a notation like '\Drupal\module_name\Controller\MyController::_field_p_rpservicename_validate' you are referring to a static function of a class.

Change  the definition of that method as follows, and your code will work.

public static function _field_p_rpservicename_validate(&amp;$form, FormStateInterface $form_state) {}


Keep in mind that a static method cannot use $this.