The ignition is turned off. The clutch is not engaged. The pressure in the vehicle's compressor unit is too low. The pneumatic valve is frozen. The magnets on the pneumatic valve are faulty. The fuse in the car has burned out.
Turn on the ignition. Engage clutch. Run the engine until sufficient pressure is generated. Remove icing from pneumatic valve. Check magnets and wiring installation. The valve can also be turned on with a screwdriver. Check the cause and replace the fuse.