That error usually appears when something is missing from your email address.
Our system uses the email address you enter as the username.
The error "Please enter a valid account username" usually appears when there is something missing in your email address, for example your email is missing an "@" sign or a "."
We have also had it happen before where there is a space before or after the email address that was typed in and that caused that same error message to appear.
Please make sure that there is no space before or after your email address and let us know if you are still having the same issue.