Hello @Patrick802
You cannot ask a guest to cancel because you can no longer host them.
If they cancel they will be penalised; they will lose their fees from Airbnb, one of their three cancellation opportunities and they are in the unfortunate position of having to find an alternative within three weeks of their stay.
I am sure you wouldn't want to put your guests in this situation of being penalised because of a host cancellation..
As an alternative could you not ask a friend or family member to manage the listing while you are away - as this means you won't inconvenience your guest as you will still get the income.
Alternatively you will need to cancel the booking. Airbnb will then give your guest a 10% credit to help them find someone else.