I believe I can help you but must first find out what model/make, and be more specific about "doesn't work", please
This is a Sharp XE-A406 cash register.
The problem are there is no tax added amount after hitting subtotal key and receipt print come out for every time when I hit the open key.
So, you have either lost the tax rate percentage or the taxability of the department key used to ring that item. If you have a manual, start by making sure your batteries are fresh, re-program the needed items per the manual.