Why does the ambient temperature constally reading -20 and my ac doesn't work?

If the outside temperature is reading an incorrect temperature such as -20, the car will automatically put itself into defrost mode. Which will in turn just blow warm air.

1:- Check for a faulty temperature sensor, which is located at the front of the passenger wheel arch behind the panel.

2:- In my case somebody had done a bad DIY job on the wiring for the outside temperature sensor and the wires had come loose.

Once your car reads the correct temperature, your AC should work fine.