I need to cancel a reservation

I am new to booking.com and neglected to black out the unavailable dates upfront and need to cancel a reservation that was made for 27 Dec 2022. I requested the guest to cancel and appologised for the inconvenience caused but she has not cancelled as yet. Since it is 10 months in advace i will appreciate help as to how i am supposed to cancel this reservation. The customer has to paid yet