The Serial Code item redemption functionality for NC Accounts is currently not working properly, but it will be again in the near future. NCoin Cards (and old Game Time Cards) and their NCoin value can still be applied to an account; only the ability to apply item codes is not currently functional. An example of an item code is the bonus items on NCoin Cards, Twitter Giveaway masks, or the 8th Anniversary Giveaways.
If you apply an item serial code to your game account you will need to send in a support ticket (https://lineage2.custhelp.com/app/ask/) in order to have the perk or item applied to the server and character of your choice.