It’s important to provide errors when creating a post.
Disabling online to fix that error is never a solution. Disabling online allows unauthenticated users to join and really just messes up player data in general.
The fix for unable to verify username would be here:
Alternatively, if that’s not the problem, Microsoft’s auth servers have been buggy in some regions the past couple days. If that’s the case you’ll just need to wait it out.