What is the "red button" on my circuit breaker
If the condition that made it trip in the first place is still there when you go to reset it, it will not appear to reset, because the problem is still there so it is essentially immediately seeing that problem and releasing the trip mechanism. There are two kinds of trip mechanisms in a breaker, thermal, which takes time and is there to protect against overloads, and magnetic, which is instantaneous and meant to protect against short circuits. So it appears your A/C unit has an internal short circuit, hence the suggestions that you get it looked at.