A warm welcome to our Community Forum sartzy and sorry you've been unable to register your online account so far.
If you've checked that the details are correct, can I ask what page exactly the error pops up? Can try clearing your browser cache and trying again with a different email address? Also make sure your password follows the guidelines and there are no double digits such as 22 or PP just in case.
Do let us know how it goes so we can take a closer look for you.
When I joined in April I had the same issue I had to use 2 email accounts 1 for Virgin Media and a separate email for Virgin Mobile as I got the Unlimited Oomph deal and you have to log in to them separately.