I'm seeing this "not eligible" error as well. It's sad to see that this issue has been persisting for a year now. I tried 5 times with two credit cards, from two different computers. A support agent told me to go into my "account settings" then "security" and to "log out of my other sessions". Aparently, every time I logged into the AirBnB website, it kept me logged in to that browser? I don't know.
I followed the advice of the support agent, logged out of all other sessions, and tried again. Still no luck. I've given up at this point, just going to buy a different gift.