You can target any player with Abzan Advantage, even if that player doesn't control an enchantment.

You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.

The enchantment will have already left the battlefield when you bolster 1.

Bolster itself doesn't target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain's bolster ability.