This means that you currently did not settle(batch close) for your credit card transactions and you'll need to settle them to process with the register close.
Go to the credit card transaction settle by pressing CREDIT CARD from register close.
Press Settle Mode to change to settle mode and use SETTLE button to settle. You may have to CAPTURE ALL first, if you have input some tips.
When settle confirm prompt comes up, press YES to confirm.
After all the credit card transactions has been settled, you'll be able to do the register close.