How can I mark this key as not valid?

I believe it will become available after the seven day window. I saw below you commented on the GA page already so beyond commenting on the givers Steam profile or adding them all you can do is wait.

Well, you have to wait for 7 days, and then you have a whole month to mark it as "not received" before the option is gone forever.

try contacting on their steam profile, no PM option on steamgifts

Not really. I asked because I have seen many, many, way too fucking many people just mark something not received without telling what their issue was.

You can try to add him on Steam or just wait until the giveaway is 168 hours old; the not received button will activate then. A simple week of wait is nothing extra and that game will have plenty more giveaways after it, so luckily you won't miss out on much. =)

They'll get notified about the comment, comments are like pm, except it's public, but better contact the user on Steam as suggested.