So my AC stopped working and I assume it had some issue with the refrigerant. So I bought the can you refill it, however when I go to refill it (After turning on the car, turning the AC onto max, and making sure the belts are turning) I find that I can't get it to fill. The pressure stays at the default when I connect the valve and when I press the handle to let the refrigerant in the pressure dial drops to 0PSI.

I went to an auto-body shop and they say they think there is a problem beyond the intake of refrigerant and offered me to fill the tank and run a diagnostic for $120. I wanted to know if there are any steps or actions I can do to try and save myself from having to pay for that?

I would be happy to answer any question. Thank you very much for assistance.