Bigbasket.com — regarding offer

Address:

North 24 Parganas, West Bengal, 700136

There was an offer of 20 percentage cashback if payment for the ordered item is to be done through freecharge app. I have done the payment by freecharge but no cash back is credited in my big basket wallet. And i have applied an additional coupon of rs 50 off (That coupon is applied). Could you please tell me the reason behind this issue?
Kindly have a look on the issue mentioned above.