Board Thread:Support Requests - Getting Started/@comment-174.64.136.59-20200415145552/@comment-34888194-20200415160825

Even if your username is not taken, if it is pretty close to a registered to a username that is already registered, then it's still disallowed, to prevent impersonation.

Is the problem only from the username that prevents you from registering? As TortoiseCat said, you should provide your username so we can identify if it is really the username problem that prevents you from reigtering.

A similar thread is here that may solve your issue.