AC system 199 chevrolet Tahoe on 1999 Chevrolet Tahoe

AC system picture showing low presure valve

Asked by for the 1999 Chevrolet Tahoe
i have a 1996 chevy tahoe. the ac nor the heating will work properly. the only thing that comes on is the fan and it only blows on defrost. it will not switch to the other settings. the heating and ac will not come on at all. it is producing no heat or air conditioning. what is going on?
2 more answers , 1 more comment
The hose, pressure valve will only install on the low side. Add refrigerant with the air conditioning on and set to high -- engine running. Add enough to get the pressure gauge in the green area.
Thank You for the reply