Athletes should be able to register for your competition if you have "launched" it (aka published it) and the registration period is currently open.
Follow these steps, to check it:
Verify you published your competition:
Verify your competition registration period is currently open:
If the registration period is open, and athletes still can’t register, please ask them for a screenshot of the specific error message they may see on the screen before contacting us through the in-app support chat.
Without sufficient relevant context, we may not have enough information to resolve the matter.