Hello, I faced the exact same issue but I contacted the nmat helpline and they confirmed that it takes upto 48 hours for payment realization

Once its done you will be able to schedule the exam

Don't worry 

Hi Aditya! Check if you have logged in to the official website with the given credentials. If not, then you have to first login to the website in order to schedule the test. If it still doesn't work, logout from the system and login once again. Despite these steps if you are unable to proceed towards scheduling the test, then contact the exam authorities by calling  +917863008300 – Monday to Friday, 9 am to 6 pm (except holidays) or write to nmatsupport@gmac.com. Hope this helps.