When you try to charge the guest's credit card and it is refused, you can mark the card as invalid. The system will communicate the invalid card event to Booking.com who notifies the guest and asks them to check the issue and correct the credit card details.
> Note that this feature work only for Booking.com reservations.
To mark the credit card as invalid, open the problematic reservation and go to the Reservation Details tab. There you can find the Invalid Credit Card button which marks the guest's cc as invalid and sends this information to Booking.com.
You can check if the information about the invalid card was sent and accepted by Booking.com in the reservation's History tab.