AC blows hot compressor clicks on and off like empty but not

So a little history on the explorer; the engine was re-manufactured 10,000 miles ago and a new transmission was built and installed 1,000 miles ago. I didn’t rebuild the transmission, and new one was built and put in. So when I turn on max air I can look under the hood and see the compressor clutch click on and run for a second then kick off. That usually means it’s out of Freon. I tried to charge the system and couldn’t get a pressure reading at all, and to my surprise, Freon is already in it. What’s going on here?