You can drive a car with a bad catalytic converter but eventually the catalytic converter may become so bad that it could cause engine stalling, loss of power, hesitation and may even cause engine damage in rare cases.
A properly working catalytic converter is essential to engine performance and when the catalytic converter becomes clogged the engine cannot breathe correctly through the exhaust.
A bad catalytic converter could also cause your vehicle to fail emissions testing if your state or county does emission testing.