You are on the Accounts Details page and the page keeps telling you that "Your VPN account username looks like this rick_morty.com@invizbox" as in the image below:
This is most likely due to a couple of characters having been badly entered.
In the example above, the user entered an @ symbol (at) in the first part as it feels very similar to his email address. But the VPN username he received was in fact "myname_gmail.com@invizbox". You can notice that the @ symbol from his email address has been replaced by an _ symbol (underscore).
Have a look at the VPN username you receive and enter it carefully in the field, it will be valid (and if you can, copy/paste it to avoid mistyping any character)