This AppID is configured incorrectly by the developer on Steam. According to the Steam API, it contains DLC only, which means that only people who own the base game can enter. You will need to delete and re-create this giveaway as the base game if you want to give it to someone who doesn't already own the game.

??

Yup, pretty much like perikorozo is saying

I don't own either the game nor the DLC and I can't enter, it says "missing base game"; which means only people with the game, can enter (to get a deluxe edition, base + DLC)
Is the ID which is configured wrong

say with steamgift staff, i'm register the giveaway correctly