Hi!! WELCOME :jumping: :cheer: :jumping:
A. Percula Clownfish develop their tail stripe last and sometimes it's late, especially with the Onyx version. If you actually have the A. Ocellaris and it's missing its tail stripe then it might have been malnourished when it was small. Most conscientious breeders will destroy malformed Clownfish. If its jaws come together properly and it otherwise looks good then it will be fine.