Same problem here. I contacted AirBnB but they didn’t fix it yet. In fact they sent me a reminder to do something that their system can’t handle! The mind boggles!
Basically you can't add your tax ID unless you are a Spanish National - because foreign residents' tax codes have a different format and airbnb's algorithyms can't handle that. They suggest replying "no" to that question and then adding your NIE. This is what they call "the workaround" - which as someone who's been declaring holiday rental (and other) income here in Spain for many years is extremely irritating. They claim there are "a few" cases like ours - but I suspect, logically, there must be thousands of us.
I guessed that was the case and in fact did exactly what was suggested (NIE added). It was a bit annoying that AirBnB didn't own up to their mistake and to add insult to injury the sent me a reminder asking me to do it - even though I had already alerted them to the problem, and they had denied it was happening!