If you have an auto-renewing subscription, you won’t be able to redeem a Gift Card/Coupon Code while your subscription is active.
If you’ve purchased a one-time Pro subscription that does not auto-renew, you can redeem your gift card/Pro Coupon code from the "Activate Pro" section under the JioSaavn App Settings on your Android device. The subscription days will be added to your account and you'll be able to enjoy all subscription features for an extended duration.