Action needed-update your user name

This is causing such a huge amount of stress I have tried changing my user name multiple times and it keeps coming back saying it isn't valid I did everything I was supposed to do. When I put the information in it gives me a green light saying it is available but then I click submit and it and comes back with an error saying I didn't do it right I have tried a minimum of 6-8 times. I should never have added a server on there.