We did not get here so it seems like the username of the credentials does not match the GitHub user name. You are supposed to have a username/password credentials item with username set to the GitHub account name, and password set to an access token.
If that is the only problem, this should be downgraded and marked with the diagnostics label to give the user some hint what might be wrong. (Only a hint, since we cannot know whether they meant to type in a user name or an organization name. We might however be able to determine that their credentials do not correspond to an actual user; that could be a form validation warning in the configuration screen.)