Hi 

Many candidates are facing this problem so you need not to worry.I believe that since the exam is scheduled in some time a lot of  candidates would be trying to download the admit card at the same time, and so the interface might be facing some technical issues.. I would suggest you to wait until the end of the day and try downloading your admit card again. And also make sure that the payment is paid fully and not partially.
Even If issues still persist, please reach out to them and mail them your issue at mat@aima.in or call them at 011-47673000.

Hope this helps.

All the best