Hii

This problem is faced by many of the students who are trying to complete there final registration. The problem must have been reported to the authorities but there is no official information regarding the technical issues as of now. You can try to login again to your account after clearing the cache of the browser and then see if it works. Also it might be the issue witht he bank so you can wait for some time say till tomorrow.
Even then if the problem persists, you can try to do the payment again and carry on with the registration process.
Also you can report your issue to the authorities on the following email - aiims.mbbsexam2019@gmail.com

Hope this helps
Thanks!!