Target player sacrifices an enchantment. Bolster 1. (Choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)
You can target any player with Abzan Advantage, even if that player doesn’t control an enchantment.
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.