Are you sure the name has already cleared? It might say that's it's SET to become inactive, but it actually isn't yet.
Yeah, that means that it should be cleared by now. Is it giving you a specific error when you try to make the pet?
Edit: Also, are you trying to use any special characters in the pet name? It was changed awhile ago so that you can't use them anymore.
i just went and adopted a pet that was on an inactive account
so whatever it is, its not site wide
If it's not due to a special character in a pet's name, is the account owned by an NPC (green icon)? If so, you cannot take username/pet names from NPC accounts.
If neither of those are the case (special characters, or NPC account) then I'd file a ticket about it. The account might be glitched or something (it happens) and a UA might be able to get it fixed for you (: