We need to ensure that GMC Numbers are only used once since they are unique to you!
A common cause of the Registration Tab not allowing you to progress is that the GMC Number entered has already been used in the system.
Normally this is caused by have two email accounts set up for the same person, one with the GMC number connected, but perhaps not logged into recently, then a new account started under a different email.
If you're experiencing this issue;
Please get in touch with the Support Team: firstname.lastname@example.org or 0333 0111 899. Let them know your name and GMC number you're trying to use, and the email you're logging in with. If you have a duplicate, let us know the email you'd prefer to use and we'll set it to that and deactivate the other.
You could try logging in with a different email that you may have used, but if this works, please let us know so we can deactivate the other email.