Did you find the solution to your problem? I did and hopefully this will help. Your pictures need to be on landscaping mode. So make sure when you upload photos they are on landscaping mode. If not, you can still change them on the AirBNB app but sometimes it cuts the photo too much so I ended up modifying the picture outsides the app to landscaping mode and upload it again.
I also am having that issue. I thought it was possibly temporarily not working, however, have been trying to change the cover photo for about one month, now, and still not working. Has anyone been successful with this?