Replace the spaces with underscores. I know it’s not terribly intuitive. In future we’ll probably disallow spaces in usernames.
So: @airborn_again [misspelt deliberately to avoid annoying airborne_again]…
Thank you!
From here
1st one is easy: there is an extra space

The @ch.ess I can’t find at the moment. Some “privacy minded” people use weird characters e.g. 0 instead of O to make their username hard to search for, but this is not one of them. Maybe there is a bug in the software preventing the @username format to work with usernames containing non-alpha chars…
Links fixed.
(why would I change my username, just because some software functionality can’t handle it?)
The issue is that a lot of people joined up in 2012 etc with usernames containing spaces etc, which the software should have rejected but didn’t.
Now we have to pay $50/hr to get these things looked at.