It is fine and I fully understand that (it can happen with DLCs). Unfortunately I am not able to say which DLC it is as I own all of them for this game missing only this one. Of course you can use that key later (if you will get this game in the future or if some of your friends have this game and missing DLCs) or you can drop it somewhere like discussion for orphan keys or some giveaway description as maybe someone would be glad to use it for himself.
You have 3 options here.
1) You can get DLC somewhere else to provide the working one key/gift.
2) You can choose to not get right key and accept not received mark (as you are new it can be a bit not welcome thing).
3) You can ask support to delete this giveaway - I approve that (so feel free to ask support to delete this giveaway to have clear shield).

Have a good rest of week and have fun. :)