The recurrence of siimilar glitches  reported by many candidates show that there is a technical fault at their end. There have been issues logging in, trying to pay and then proceed for further steps etc. AIIMS has not released any official notification regarding the dealing with this issue. Most likely, it will be resolved eventually if not immediately.
Please close all tabs and logout  from your mobile or any other device where you might have accessed the account. Moreover try again using a different browser, clear cache, clear browsing history, restart, re-login and then try it again.  Maybe if nothing works, it is better to write directly to them or get on a call with them.