Booking.com reservations cannot be canceled by an owner in MyVR. All modifications/cancellation of Booking.com reservations must be initiated by the renter on Booking.com.
Any issue with the reservation that cannot be resolved by a modification will require owners to contact Booking.com customer service.
MyVR suggests messaging the renter and asking them to request a cancellation through their booking.com portal.
More info on cancelations from booking.com can be found here, https://partnerhelp.booking.com/hc/en-us/articles/213302445