Welcome to Steamgifts! You can't give away Freebie because it has been given away free in large quantities-- there's no obvious way you would have known this, but that's why the game didn't appear in the list. From the Guidelines:

Giveaways should accurately reflect the game being given away

You'll need to delete this giveaway with the Delete Giveaway link on the left nav, or likely be suspended for a misleading giveaway (and giving away free games). Thank you for trying to give away a game, though!

By the way, if you still want to give away the game, you can post the key in this thread and somebody will snap it up in a hurry! =)

Gracias

Cheers!