@Lisa5993 Guests don't need permission from a host to cancel, it is never a matter of it being allowed or not allowed, regardless of the cancellation policy in place.The issue is how much of a refund they would get. And a host always has the choice to refund in full, regardless of the cancellation policy.
But if a host can't host because their area is in the midst of a disaster zone, or a fire, hurricane, etc. makes it impossible to access the property, you don't ask the guests to cancel- it's up to the host to cancel.
You should read through the Extenuating Circumstances policies to understand which situations would qualify for a penalty-free cancellation. But cancelling under that policy requires that you need to contact Airbnb customer service, you can't just cancel on your own or you'll receive penalties.