If an error occurs during your registration this will usually be a temporary error with a server.
Try closing the browser down and opening a new one and trying again. If the error persists it might be that there is a technical glitch which will usually resolve itself and you should try again a bit later on.
However, if you continue to receive error messages when trying to register you can submit a query to our Online Service team via our webform here with the details of the error message you receive and at what point in the registration you are receiving this message. If you would rather email or phone them, their contact information is available here.