where can i find booking.com phone number so i can cancel a booking i have just received in error.

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