Hi David, You've hit your first learning experience as a new host!
Airbnb takes cancellations very seriously because guests get super-dissatisfied if they think they have a confirmed booking and then hosts pull the rug out from under them (whether it was well intended, as in your case, or because they thought they could make more $ with another guest, didn't keep their calendar up to date, etc.)
When you cancel, Airbnb does a number of things:
https://www.airbnb.com/help/article/990/how-do-host-cancellation-penalties-work
Including blocking your calendar. You can't unblock it, but you could try to sell those dates through another platform.
There's a lot to learn as a new host - keep checking out the forums and Airbnb help articles so you can avoid other pitfalls. It'll get easier as you learn the ropes 😉