can find a game in giveaway

It may have been exploitable at some point and administratively removed from the giveaway options. Contact support if you believe this to be an error.

hmm then can i give it away on the forums instead ?

Contact support and ask them to add. It's not on the exploited key list in the F.A.Q. so it may be that it was just overlooked. I had a similar issue with one of my own giveaways (Just Cause Collection) and they added it pretty quickly.