Message the guest and explain you need to cancel. then go into the Reservation and click 'Request to cancel' this will send to guest and they confirm. Then you don't need to contact booking.com who will charge you if they need to relocate. Here are phone numbers
I found this buried under Partner Help > Your Reservations. So many of us likely have never seen it. This should have been pinned and featured on the community
Message the guest and explain you need to cancel. then go into the Reservation and click 'Request to cancel' this will send to guest and they confirm. Then you don't need to contact booking.com who will charge you if they need to relocate. Here are phone numbers
https://partner.booking.com/en-us/help/where-you-can-reach-us