They’re not asking you to join another service to get to join AdRev. They’re simply giving you alternatives as they won’t accept you. I guess they checked if you were registered with AudioJungle. I’ve seen this happen before.
Also, as PurpleFogSound said, make sure you registered here http://cid.adrev.net
I guess this has to do with the new policy from YouTube’s side that AdRev need to make sure if there’s a license in place before making a claim on the video. Which means a lot of extra work for them when it comes to stock libraries such as AJ. I’ll see if I can find the post from Alumo where he got an answer from AdRev regarding the new policy.