If your account was suspended during the last move (as in before the move began), then the account wasn't copied over. It was made known via the News/Announcements forums that you would have to request the account be restored.
The deadline to request said restore passed several months ago; we don't have backups that old anymore, so if the account wasn't copied over, we can't re-copy it now.
It's also possible it was suspended for inactivity (failure to login to the forums once every 14 or 30 days depending how old the account was) and then terminated 14 days later when an unsuspension request wasn't entered through the account panel.
You're still welcome to create a new account, but we can't recover the old one at this point - the old backups from when we did the major maintainence have long-since been erased and replaced with more up to date ones :/
Edit: Or what descalzo and calistoy said. If you've never actually -had- an account, then that's why it's showing an error.