Custom form validation is missing

I have a registration form :

<div class="register-error" id="register-error">{{ form.status_messages_user_register_form }}</div><div class="row"><div class="col-sm-6">{{ form.field_first_name }}</div><div class="col-sm-6">{{ form.field_last_name }}</div></div><div class="clear"></div>{{ form.account.mail }}{{ form.account.name }}<div class="row"><div class="col-sm-6">{{ form.account.pass.pass1 }}</div><div class="col-sm-6">{{ form.account.pass.pass2 }}</div></div><div class="clear"></div>{{ form.form_build_id }} {# required #}{{ form.form_token }}{# required #}{{ form.form_id }} {# required #}{{ form.actions }}

Now i want to add custom validation for the registration.I have a custom ajax handler for registration using form alter:

$form['actions']['submit']['#ajax']  = ['callback'    => 'custom_register_form_ajax_validate','event'       => 'click','#attributes' => ['class' => ['use-ajax',          ],         ],       ];       $form['#attached']['library'][]      = 'core/drupal.dialog.ajax';       $form['#attached']['library'][]      = 'core/drupal.ajax';       $form['#prefix']                     = '<div id="email-valid"></div>';

And this is my function :

function custom_register_form_ajax_validate(&$form, $form_state){$response = new AjaxResponse();  $form_state->setTemporaryValue('entity_validated', TRUE);  $firstname = $form['field_first_name']['widget']['#default_value'];  $email = $form_state->getValue('mail');  $pass1 = $form_state->getValue('pass1');  $pass2 = $form_state->getValue('pass2');    if($firstname == ''){         $text = '<h5 style="color:red;">Firstname is required.</h5>';          $response->addCommand(new InvokeCommand('.form-text', 'css', array('border-bottom', '2px solid red')));      $response->addCommand(new HtmlCommand('#email-valid', $text));    }    if($lastname == ''){         $text = '<h5 style="color:red;">Lastname is required.</h5>';          $response->addCommand(new InvokeCommand('.form-email', 'css', array('border-bottom', '2px solid red')));      $response->addCommand(new HtmlCommand('#email-valid', $text));    }    if($email == ''){         $text = '<h5 style="color:red;">Email field is required.</h5>';          $response->addCommand(new InvokeCommand('.form-email', 'css', array('border-bottom', '2px solid red')));          $response->addCommand(new HtmlCommand('#email-valid', $text));    }}

I want to validate "Firstname is required" condition.But when i submit and validate the form,always showing "Email field is required".

How can i solve this issue?