2000 Chevy Silverado no a/c

Thought I might be low on refrigeration,
so I bought the charging hose & gage and 134a.
When I turn a/c on, I can hear the clutch engage
but it won't take any refrigerant. No cool air!
I am not sure I have attached hose in the proper place. What else could be wrong? I pulled the
low pressure switch, it dis engages, I put it back on
and it re engages?? Please help