What to return from form AJAX Callback
return $form['elements']['status'] is correct.

you need to create your form elements in 

public function buildForm(array $form, FormStateInterface $form_state) {

$fields = $form_state-&gt;getValues();
if (!isset($fields['status']){
   $form['elements']['status']['#prefix'] = '&lt;div id="ajax_status"&gt;';
   $form['elements']['status']['#suffix'] = '&lt;/div&gt;';
   $form['elements']['status']['#disabled'] = myfn_disable_attendance($form_state);
}