This means that you currently have one or more open tickets and you'll need to close them to process with the register close.
Check the open ticket from Sales (or map) screen > TICKETS section and close via paying 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.