While you are doing the Register Close, you may find Tickets has orange highlight and the Save button is disabled.
1. Sales screen(map screen) > Menu > Register Close > Tickets has orange highlight
This means that you currently have one or more open tickets and you'll need to close them to process with the register close.
2. Check the open ticket from Sales (or map) screen > Tickets section and close via paying or the tickets.
Press the Tickets and you will see all open tickets which have not been closed.
You may use Pay function to pay the ticket on the spot.
After closing all the open ticket, you'll be able to do the register close.
If you do not want to close the open ticket but still want to do the resgister close, see: Register Close - Skipping Unsettled Ticket/ CC/ No Tip