Answer from AirBnb support (have not tried the recommendation yet):
Hi Chantale,
I am sorry for the delay in response.
I would like to apprise you that if you log in to your account on computer using a chrome browser, you would be able see the calendar accordingly.
The dates are not blocked and it's open. The issue is not at our end, however, the API software is throwing the error in this case.
Request you to get in touch with the manager of the account or the App for your assistance as I am attaching a few screenshots below for your reference.
You can also go through the below Help Center article for your understanding:
https://www.airbnb.com/help/article/2233/how-do-i-change-my-sync-settings-for-api-connected-listings...
I hope the information provided was helpful to you. Please feel free to let me know in case you need further assistance. I am just a message away.
Have a pleasant day ahead!
Best,
Rasu