Hi. I would need to know the brand, model number, and what the error message says. It sounds like you haven't finished in another mode before turning the key. They won't let you turn to Z mode if you are still halfway through a sale in REG mode for example. Turn back to reg and see if you do a 'no sale' or just enter a sale, press CASh and then try it in Z mode again. If you still have no luck, let me know the brand, model, and error.
Mark, October 2008