I’ve searched the other similar answer but still no luck.

I have content type Station with field Province and City.

Form alter:

function mymodule_form_alter(&$  form, FormStateInterface $  form_state, $  form_id) {   if ($  form_id === 'node_station_form') { //    kint($  form);     $  form['field_province']['widget']['#ajax'] = [       'event' => 'change',       'callback' => 'mymodule_callback_example',       'wrapper' => 'edit-field-city-wrapper',       'method' => 'replace',     ];      $  form['field_city']['#attributes']['id'] = 'edit-field-city-wrapper';      $  options = ['_none' => '- None -'];      $  user_input = $  form_state->getUserInput();     $  province = $  user_input["field_province"] ?? NULL;      if ($  province) {       if ($  province === 'province_a') {         $  cities = [           'province-a-city-1' => 'Province A City 1',           'province-a-city-2' => 'Province A City 2',           'province-a-city-3' => 'Province A City 3',         ];       }       else {         $  cities = [           'province-b-city-1' => 'Province B City 1',           'province-b-city-2' => 'Province B City 2',           'province-b-city-3' => 'Province B City 3',         ];       }       $  options += $  cities;     }      $  form['field_city']['widget']['#options'] = $  options;   } }  function mymodule_callback_example($  form, FormStateInterface $  form_state) {   return $  form['field_city']; } 

The dropdown City successfully populate based on Province, but when I click submit form, there are error "The value you selected is not a valid choice"

What did I miss?