Welcome to our Community and thanks so much for your first post - I am sorry that you've had some issues with your online account. From locating your account from your forum information we're not able to see that the account in question has ever had an online account registered.
When you register, please ensure that you're using 9 digits for your account number and that the spelling is the same as what is listed on your bills (This is worth checking encase there is a spelling error that you may not have previously notices - the spelling must match what we have listed on the account exactly)
Thanks for using the forums to get this issue looked into, I am sorry you're still having this issue with your online account. The information you gave about moving home was very useful as this may actually be the cause of the problem!
When customers move addresses, they are moved onto a new account number. But sometimes their online account details are not transferred from their old account to their new one, thus the webpage throws up these error messages.
We can help transfer your details onto your new account here on the forums, this is called an M&T or Move and Transfer. Sometimes it could need an IT ticket creating if we encounter a problem, but in most cases its an easy fix after 24 hours!
I will send you a PM now so we can have a look at this in more detail.