I'm trying to diagnose the patients, but when I move over the patients, it says "Press E to diagnose patient", and pressing "E" does nothing at all

You don't have the quest for it yet.

I'm sure I do, because he asked if I had any medical experience, and I had like 20 so the response was "I do" and he said to go take a look at them. Now the only dialog option I have with the doctor is that I am not qualified to diagnose the patients. My medical is currently like 40-45