@Sharon349 When a host cancels confirmed bookings, one of the penalties for that is a permanent block of the dates that were cancelled.
What you should have done was contacted Airbnb at the time, providing documentation of your dad's condition, which they may (or may not) have considered to allow penalty-free cancellations for under the EC policy.
All you can do now is contact them and explain why you had to cancel, to see if they will take pity on you and unblock the dates. But don't hold out much hope.