UPDATE: the issue has been fixes, today August 13, 2020 Airbnb released new app update. Just update your app to the latest version (or delete and install the app) and the calendar issue will be fixed automatically.
Airbnb calendar block/unblock option in the app is currently (August 8, 2020) having some UI errors. When you block dates the Edit button does not show up.
Solution: try to block any dates and check the lower-right corner, you will see a fraction of a green Edit button. Depending on your device screen size you may not see any remains of the button at all, in this case you will need to switch to a browser version of Airbnb for dates blocking.