Welcome to the forum!! Please advise your location, as it is possible that you are not on the correct side of this forum (this is for 1989-1998 cars, usually). However, I can give you a bit of information. In your car, you have two oxygen sensors, one on the exhaust manifold before the catalytic converter, and one on the exhaust, just after the catalytic converter. The front O2 sensor normally switches between 0 and 1 volt, cycling up and down. The back O2 sensor - if the catalytic converter is working correctly, usually gives a steady reading and does not cycle. It's steady reading is based on the fact that the catalytic converter is reducing the exhaust gases, as it should. When the front and the back O2 sensors give the ECU a similar reading (which means, of course, that the back one is not giving a steady reading but is showing that the catalytic converter is not working properly) you will get a code 420.
This can be caused by many things - the front O2 sensor not working properly, the rear O2 sensor not working properly, the catalytic converter not working properly, an exhaust leak, poor ignition timing, etc.
If your problem is because either of the O2 sensors are not working properly, or their wiring is not good, or you have an exhaust leak, then yes, you can drive the car. If your problem is that the catalytic converter is blocked up, so that the exhaust gas does not come out easily, and the converter gets very hot and turns red because it is blocked, it could be a fire hazard - however, you would know this, as the car would run very, very poorly (as the engine would be unable to 'breathe' properly). So, basically, yes, I would say that you can drive the car.
In the meantime, please advise your location - you can get better info if we put you into the proper section.
'91 Tracker 8V, 1.6L, 5 sp, 4wd, 2dr, conv, CAMI, horribly rusty
'97 Suzuki Vitara 8v, parts car - worth its weight in gold.....
'61 NSU Quickly - 150mpg