Hi Kate08
Thanks for posting and welcome to the community. My apologies for the online account issue.
Having checked the system, are you trying to log in to the online account you had at a previous address? If so, then you'll need to do a Move and Transfer for this. What that does, is basically move the online account from the old account to the new one, timescale is 24 hours.
If you're the account holder, as this is something only an account holder can do, let us know and we'll be able to assist further.
Kind regards,