I believe you are talking about this cashback promotion in India, is that correct? Please take a look at the information in the FAQ on that linked page to see if the answers there resolve the issue. If the voucher is in your account, that FAQ will show you how to see it and how it is automatically applied to your next purchase. If there is no voucher and the cashback was not applied, that FAQ should help you to understand what requirements there are in order to earn cashback.
Advice is voluntarily given. Kudos / Solution appreciated if I helped you.