@Luke486 Are you quite sure you thoroughly read through everything in the listing information, clicked on the amenities list, etc? Often a host has no control over when a pool is closed, if it is a shared pool maintained by the complex and the host will say somewhere in the listing that the pool may be subject to closure with no notice. So make sure this isn't the case- that you just missed reading something.
It is unclear whether you are now staying at this place or whether this is an upcoming reservation, and if it is upcoming, how did you find out the pool is closed?
If you are quite sure the ad doesn't mention the possibilty of pool closure, and not having a pool is a total deal breaker for you, yes, you have grounds for a refund. Some hosts will offer a discount for a missing amenity, so if you'd stay in that case, you could ask the host about that possibilty.