I now see the message pasted below when I try to modify a guests checkin time.
How can I change a checkin time per reservation in Airbnb? They don't seem to have that capability unless it's somehow locked by using the API.
Channel Calendar Linked
This booking is managed by the firstname.lastname@example.org Airbnb channel.
If you can, we recommend making any changes on the channel which will then automatically update the OwnerRez booking. Or you can unlink from the channel calendar and make the changes in OwnerRez, but they won't be reflected on the channel.
Ah, too bad the OR forum software doesn't support attachments--I took a screenshot. Hopefully this link works, though: https://imgur.com/a/X8IJi9o
Even if it is channel-managed, you should still be able to change the _time_ (just not the date), without unlinking the booking, as pictured below. If you do not see the same thing, I'd suggest probably reaching out to email@example.com for them to look at your account specifically.