Can't search game for a giveaway

Very likely given away for free in a massive quantity.
Someone will be along shortly to repeat that and perhaps offer more detailed info.

Since I have it, I would never have bought it, have not bought any recent bundles and didn't win it, I'm guessing it was free at some point which removes your ability to give it away.
Edit: Confirmed, it had a mass giveaway about 6 months ago

Well that's a bummer, I just bought it for 14 cents to give it away here.

You can find the list of games that were once free in this thread, so you can avoid situations like this.