How can I create a custom ajax callback command?
It seems like you have a namespace error(s). The error happens in a file you didn't mention RegionWidgetType.php. It's trying to use Drupal\Core\Ajax\ReadMessageCommand which as I understand it is something you made. Which means that the namespace should definitely not be \DrupalCore\... for the auto load to work.

The name space of your custom files should be Drupal\MODULE_NAME\folder\structure