You have to wait for 15 days before you retake NMAT exam from the last time you attempted it. You can retake NMAT exam twice during the 90 day test window. So though you can register after 24 hours for a retake, you can only appear for the exam after 15 calendar days. The confirmation mail depends on the slot availability. So please check if you fall within the 15 calendar days time slot and also 3 attempts within 90 days. You can also reach out to the helpdesk at: nmatbygmacsupport@merittrac.com