This is typically the result of a registration tied to the account owner rather than the athlete. There are several ways this can happen; however, there is usually just one fix. An athlete needs to be removed and then re-added to the roster by the team or an administrator with permission.
If you are a Team Administrator with "Full Access" or "Limited Access - HQ Tools" permissions or higher, click here for instructions on how to remove an athlete from the roster and/or here for instructions on how to add them back.
If you are the athlete's parent, you must contact your team or league admin for assistance.