I would speak to the management they should have alerted you to this amenity being closed @Catherine3832
If the guest is already there offer a discount if they want to stay or penalty free cancellation if they don't . I wouldn't offer a credit for another time .
make it clear up front in your listing when the pool is closed.
contact guests with upcoming bookings offer a discount or free cancellation while the pool is closed
by the way do look at the Airbnb Help website it has lots of useful information on managing your listing