Hello @Alice129,
You get the opportunity to give the reason when you click to cancel a particular reservation.
To cancel a reservation:
select "Dashboard" from the "Host" menu when you log in to the website
then select "Your Reservations" in the black bar at the top
then select "Change or Cancel" for the desired reservation
then select "Cancel Reservation"
You can then give the reason and see information on the penalties that will ensue if the reason isn't on their list of extenuating circumstances.
If you wish to follow through with the cancellation it's probably best to contact Airbnb to ensure that your reason will be accepted.
They may require documentary evidance.
Contact-Airbnb-A-Community-Help-Guide