Are you allowed to drive a car if the parking brake is the only working brake?

No, all motor vehicles must be equipped with two working and mutually independent braking systems: a service brake (also known as foot brake) and a parking brake (also known as hand brake). You must never drive a car if the parking brake is the only working brake.

However, if, for any reason, the service brake stops working while driving, you can try to stop the car using the parking brake