The game have already activated,

I definitely wouldn't, did you receive the game? No. So why approve? I would report the giveaway creator and/or you could try to confront him about it.

No, talk to the guy who created the giveaway.If he doesn't give a valid one, mark as non-received

You didn't get the game in your library, right?

If so please contact the support.

No. You should ask the giveaway creator for a new game, one that works. If they refuse, contact support.

No as you haven't received the game.
You should contact the creator of the give-away and if you cannot resolve it with him then report him for a fake give-away.

Get him to buy you one in the steam store and gift it to you.

Thank to all. The giveaway creator send me another key and it non-activated.