That probably means it's been transferred before, which would mean that it cannot be transferred again. Anyways, if you simply took over his account, XenForo doesn't consider you the owner of the license. The original owner can take back possession of the license if he ever feels the need to do so, so you can't really guarantee that license use would be included. That's guaranteeing that the forum will always be attached to this particular XenForo license, and that's something that you can't guarantee, if you aren't the actual owner of the license (in XenForo's eyes).