You're in the BIOS- there is no sleep function. You've clearly already figured out that you can't just require() the os module, because that's provided by OpenOS, which you don't have here.
The OpenOS implementation of os.sleep is here. Notice that it just polls events until it reaches the timeout. You could do that yourself, but now you have to figure out what you're going to do with all the events you get, since there doesn't seem to be a way to pause execution with computer.pullSignal without actually receiving events.