Hello @Rosana32,
If you receive payments within the Airbnb system you can't set a cancellation policy other than one of the Airbnb official ones.
The guest pays Airbnb when they book and Airbnb holds that money until they pay you a day or so after the guest checks in.
Airbnb holds the money and so is in control of cancellation refunds. They support only whichever of their pre-defined policies you have selected. That is, flexible, moderate, or strict. Or, automatically, long term if the reservation is for 28 days or more. They will process refunds based on the provisions of whichever of the above policies applies.
Steve.