If the guest's credit card is showing as invalid/declined, follow the steps below to report the credit card as invalid on Booking.com.
Click on the blue gear icon on the top right side of the reservation page
Select "Mark guest card as invalid"
Once you refresh this page, the payment status banner will change to "credit card marked invalid"
Booking.com sends a message to the guest requesting an updated credit card
If the guest provides a new credit card, this will be sent to Direct as a reservation modification
If guest doesn't update the credit card within 24 hours or the guest provides invalid credit card details again, the reservation can now be cancelled in Direct
When the reservation is able to be cancelled you will see "ready to cancel" above the reservation code in the reservation list view.
To cancel the reservation, select the reservation, click on the blue gear, select "Cancel due to invalid credit card"