This error means you have a customer tab open. All tabs need to be closed or moved before cashing up. If you need to keep them open for payment, you should create an account and then assign the tab onto it so it can be paid at a later date.
Open the customer tabs area by pressing the Tabs (or Tables) button next to Name Me on the order screen.
In the Tabs screen, you need to close off any existing tabs or tables. They will appear with a green border.
Press the tab to open it in the order screen and pay the tab off - or alternatively, assign it to an account for later payment. If the tab is in error you should void the items and then pay the balance of $0.00 to Hospo or Cash in the payment screen to fully remove the tab.
If the tab balance is a negative number, you will need to bring it back to being positive, by adding a small item to bring it back to positive. Alternatively you can create a misc priced button and call it "Fix Up", add it to your menu, sync menu on the iPad, then use it to add the same amount to bring it back to $0.00.
Once you pay it off to Hospo or Cash you should then be able to cash up.