If you choose an Aura, you also choose a legal permanent for the Copy Enchantment copy of it to enchant. Copy Enchantment doesn't target that permanent, however, so it can enter the battlefield attached to an untargetable creature.
If you choose an Aura and there isn't a legal permanent for it to enchant, you put Copy Enchantment into your graveyard. It never enters the battlefield.
If you don't choose an enchantment, Copy Enchantment enters the battlefield without copying anything, and it sits there with a useless ability.
If this is copying a Genju, such as Genju of the Fields, and the land this enchants goes to the graveyard, you will return the Copy Enchantment to your hand.
Copies all copiable values, to include mana cost, so if you copy Dream Leash, for example, the converted mana cost of Copy Enchantment will be 5.