While you are doing the register close, you may find TICKETS turn orange and the save button disabled.
This means that you currently have one or more open tickets that have not been settled.
To close the open ticket, go to the sales screen and on top left corner locate OPEN button.
Press it and you will see all open tickets which have not been closed
You may use PAY function to pay the ticket on the spot
Or you can open the ticket to close ticket by either paying the ticket or voiding ticket (if this open ticket was made by accident or so)