Hi Dr Spaceman, I had the same issue. It wouldn't let me link the account for a while, but it worked after I tried this, which was suggested by Jan at the box office:
You need to log in to your account and remove the new account you created, you can do this from book.nufc.co.uk and where it says 'not you' click there.
After I did that it let me link to my previous account, and then the renewal worked