Hii

Please try again after clearing the cache of the browser and make sure to enter correct registration number, RUC as well as password and the captcha that is shown there.
Also try to work during the off peak hours when the sites has less rush.
Even if the same problem persists, then try to change the password by loggin in to your basic registration form and try again for final registration.

Still if the problem is there then you can simply mail your query to the authority on the following email - aiims.mbbsexam2019@gmail.com

Hope this helps
Feel free to revert back for any more query.

Thanks!!