Can you use DELETE API instead of CANCEL? I've not tried this myself but looking in https://learn.microsoft.com/en-us/graph/api/bookingappointment-delete?view=graph-rest-1.0&tabs=http it appears to have permissions for BookingsAppointment.ReadWrite.All
NB Comparing DELETE/CANCEL reference articles, note that DELETE API does not appears to send the customer any 'cancellation' email