Please arrange alternate reservation for Booking number, ***

You have posted this on the partner forum so we cannot help with this.  You should be aware that booking.com will charge relocation fee for this.  Are you able to contact the guest and suggest alternative and get them to cancel your booking.  otherwise contact booking.com help team using Inbox icon on Extranet or give them a call see link for local phone numbers. 

https://partner.booking.com/en-us/help/where-you-can-reach-us