It doesn't work by 'user name', it works by the actual URL. For most people, since about 2013 that's been a series of random letters & numbers which don't change even if they change their channel name.
When in doubt, you can get the URL by going to your "channel page" and pressing "F6" key, copy & paste.
Or if your channel has a 'videos' tab, this often has the old style URL like "youtube.com/user/(whatever-their-name-was-back-in-the-good-old-days)