Change Field value in drupal 8 form with Ajax

Form.php

<?php
/**
 * Form controller for the bb_report entity edit forms.
 *
 * @ingroup report
 */
class FeeForm extends ContentEntityForm {

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildForm($form, $form_state);

    //Set all as empty value in province.
    $form['province']['widget']['#options']['_none'] = $this->t('- All -');
    
    //Set a wrapper for the ajax petition.
    $form['province']['widget']['#prefix'] = '<div id="edit-province">';
    $form['province']['widget']['#suffix'] = '</div>';

    //Add ajax function to complete another field.
    $form['country']['widget'] = [
        '#ajax' => [
          'callback' => [$this, 'rebuildProvinces'],
          'event' => 'change',
          'wrapper' => 'edit-province',
        ],
      ] + $form['country']['widget'];

    return $form;
  }

  /**
   * @param array $form
   * @param FormStateInterface $form_state
   * @return AjaxResponse
   */
  public function rebuildProvinces(array &$form, FormStateInterface $form_state) {
    $countries = $form_state->getValues('country');

    //Get current country selections.
    $countriesIds = [];
    foreach ($countries['country'] as $country) {
        if(is_array($country)) {
          $countriesIds[] = $country['target_id'];
        }
    }

    //Get associated province ids
    $provincesIds =  \Drupal::entityQuery('province')
      ->condition('country', $countriesIds, 'IN')
      ->execute();
    $provincesIds[] = '_none';

    //Filter provinces using selected countries.
    $form['province']['widget']['#options'] = array_intersect_key($form['province']['widget']['#options'], array_flip($provincesIds));
    return $form['province'];
  }
}