A YouTube coupon code is a credit that can be applied to a movie or TV show rental or purchase. Coupon codes can only be redeemed for the indicated purpose. For example, if you received a coupon code for a movie rental, you cannot apply the credit to a TV show purchase.
You can redeem a coupon code on a computer or using selected mobile devices.
It's currently not possible to redeem a coupon code on an Android device. However, you can redeem a coupon at youtube.com on a computer. See how to redeem a coupon here.