Board Thread:Support Requests - Getting Technical/@comment-46662447-20200818232548/@comment-25014407-20200818234549

It's not necessarily about the exact username, but it can be that a similar username has already been taken. Looking at the user table, I can find a  which already exists. I am not saying that your preferred username is too close to that one, but it could be one of the usernames that prevents you from using that. If you want to change your username (which you can only do once!), it might be better to choose another name, or to use Special:Contact to ask if it's possible.