2003 Hyundai Sonata aircon pump not kicking in

I recently had the aircon pump replaced, but only went to fill up the gas today.

The technician who was responsible for filling up, said they could not as the pump was not kicking in when you switch on the aircon. Should it not first be filled with gas before the aircon will start working, or can the pump start working without any gas, as there is no pressure to kickstart the pump.