You entered the code incorrectly.
Copy paste the Blizzard code that we delivered to exclude typo's and try to redeem it again.
Blizzard codes can be checked and redeemed at https://eu.battle.net/account/management/claim-code.html.
You or someone else already used this code.
Try to re-login with your Blizzard account, if the subscription or the Blizzard credits are not activated in your account - contact Blizzard support.
Go to https://eu.battle.net/support/en/ to contact Blizzard support.
Blizzard Balance can only be purchased and used with your country's currency.
Blizzard codes are specific to your region. If you're trying to redeem an Blizzard Gift Card that was purchased in a different region, make sure that you're redeeming it in the Blizzard Store for the same region. You can't redeem cards outside of the region of purchase. For example, an Blizzard Gift Card purchased in Europe can't be redeemed in an Blizzard Store in the U.S. When redeeming Blizzard Gift Cards, make sure to redeem the cards in the Blizzard Store for the same region.
You can use the Blizzard Code Redemption page (https://eu.battle.net/account/management/claim-code.html) to redeem all codes and game keys, including physical or digital game keys, game time codes, and pet or mount codes, and Blizzard Balance codes.
If you are still experiencing problems after following the above steps? Then please contact Support.