@sahmad Email rupay team as reply to the email in which you received code. Ask them to check/ investigate at their end and do the needful. If it is their fault (or that was redeemed even before it was delivered to you) then they will give you new coupons.
Just in case if they deny ask them to share more details, but you have to be 100% sure about it.
-----
Another point. Nykaa wallet loading sometime give problem even if code is valid. Try any sample purchase and apply that code in payment page just to test. You can remove it later on and start afresh.