Because targets are chosen before costs are paid (such as the cost of sacrificing an enchantment), Ghen's ability can't target the enchantment you intend to sacrifice to activate its ability.

An Aura put onto the battlefield this way doesn't target anything (so it could be attached to an opponent's permanent with hexproof, for example), but the Aura's enchant ability restricts what it can be attached to. If the Aura can't legally be attached to anything, it remains in your graveyard.