The Zero Price error message will appear (along with an audible beep) if you try to sell a product with a zero price that does not have the the PLU option "Enable zero price sale?" set to Yes.
This issue could be caused if someone has changed the Price Level on the POS terminal and the product is now being sold at a price level where the product has a zero price. The price level a POS terminal is currently using is displayed in the top right hand corner of the sales screen.
Resolution:
Enable the PLU option "Enable zero price sale?".
This is done via:
TouchOffice > Products > PLU Programming > PLU Details > Terminal Flags > Status Flags > Enable Zero Price Sale?
If the POS terminal is using the wrong price level, it can be changed via:
Home > Manager Functions > Sign On > Change Price Level