This usually means that you have already tried to register but did not complete the process. In order to proceed with the registration, you will need to begin the application process again from the beginning. Once you have filled out the first page (using the same information as your initial registration attempt) and click to proceed, the page will refresh with the following alert:
‘A user account with the email address you have provided already exists. You can enter a different email address above to try again. Alternatively, please enter the password for this user account below, use the forgotten password functionality to retrieve your password’
If you remember the password that you had entered originally, then please enter this password in the ‘Existing password*’ box, followed by your desired password in the next two empty boxes, and then once you click the button to proceed, you should be allowed to this time.