Not a lot of info to go off of here, but are you entering it in capital letters on Mog Station?
We posted a guide to help users redeem their codes >> here <<
Also on the PS4 side of things, it will simply tell you "Thank you for using PSN." once redeemed. You can confirm your entitlement via:
PlayStation®4
Visit the “Settings” menu of the PS4, then select “PlayStation™Network/Account Management,” then “Account Information.” Scroll down and select “Services List,” and find “FANTASY XIV: A Realm Reborn.” Selecting it will show Heavensward entitlements (such as "CE Playing Entitlement.") and the next time you launch XIV you will tie that to your service account.
