Possible reasons why you cannot redeem a Google Play gift card:
- Your Google Play account is not from the region for which the gift card was purchased
- Your IP is not from the region for which the gift card was purchased
- You try to redeem with VPN
- Your age on your Google Play account is under 18
If all the conditions for redeeming are met and you still cannot redeem the gift card than you need to contact Google Play support.