Check the release date of the PKHeX you're using. Then check the release date of that event. In all likelihood, that event did not exist when PKHeX pushed the update. (Example, PKHeX was released before the event was released) So wait patiently for Stable Release PKHeX to be updated. (Or download the development build at your own risk. Make sure you keep additional backups if you do so. There's no guarantee the development build you downloaded has added the event data in their reg