You entered the code incorrectly or the country of your PSN account is different than the country of your PSN code.
Check for any typo's, check if you entered 12 digits, check the region of your PSN account.
Login with your PSN account at; https://account.sonyentertainmentnetwork.com. Go to Account details to verify the country/address settings. The registered 'location' needs to be equal to the PSN giftcard.
You or someone else already used this code.
Restart your console and re-login with your Playstation account, if the subscription or the PSN credits are not activated in your account - contact Sony.
Go to https://account.sonyentertainmentnetwork.com and login with your PSN account. In your Dashboard you can find your 'wallet' credits. Or go to 'Media' and check if you can find your PSN Plus subscription, the expiration date of your subscription
If you are still experiencing problems after following the above steps? Then please contact Support.