The error and the explanation you provided match. Unfortunately I was unable to reproduce the issue from 47a830330f or on main. I tried it signed in as an admin and as a regular user, and they both load fine.
As a sanity check I just want to confirm you’re using the version of gamification that’s bundled with Discourse core, and doesn’t have the old, stand-alone plugin installed?
Are you able to provide more of the stack trace of where the MissingAttributeError is happening?