I am not sure a manager can self enrol into a course since they are in the course already. (In my previous post I meant that you can add self enrolment with the manager role, but that is not the same as allowing a manager to self enrol. And is pointless anyway.)
However, if they are a manager with the system role then they should be able to go into the course settings and add themselves as a student and then mark as complete. I suggest you tell them to do that?
(But it is really not ideal)
(But it is really not ideal)