Received a code from a giveaway but it's invalid, what to do?

Let it go?

You mark it unreceived. That is all, it's not exactly like "negative feedback". No one even notices your GA rep unless they are running a userscript anyway.

Chat with the user. If they are unwilling or unable to give you the game you won, you must mark it as Not Received.

First, try to resolve it with him. Tell him it didn't work and see if he's willing to provide a working one. If not, mark it as not received. He has an unlimited amount of time to provide a working key to you, so whether he has to grind on tremorgames for a year or wait for it to be bundled you can keep him on your list until it's complete.

Alright, makes sense. Thanks everybody!