You have posted this on the partner forum so we cannot do this for you. Contact booking.com help team using Inbox icon on Extranet (Option booking.com messages) or give them a call see link for local 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
Try and contact guest and get them to cancel.
You have posted this on the partner forum so we cannot do this for you. Contact booking.com help team using Inbox icon on Extranet (Option booking.com messages) or give them a call see link for local phone numbers.
https://partner.booking.com/en-us/help/where-you-can-reach-us