Did they released admit card already? I don't see any notifications or link from their website aiimsexams.org .. Where did you find the link? May be you are trying to login to registration page.

It could have happened because of some issue on the server. Maybe there were a lot of people on the server when you tried to download your admit card, try downloading it again after some time. This should work, and you'll get your AIIMS examination admit card.