There currently are 5 reasons of why this could be happening:

• Your steam gift inventory settings are set to private.
• We are not currently accepting that game, either because the game banking manager decided so or because it cannot be banked.
  • If it's the latter, the gift will be using the Steam Gift placeholder image. Example.
• We have already hit the stock limit of that game
• Your game is region locked.
• You are trying to sell a game that has been added to your Steam Game library instead of a gift that is in your Steam inventory.