Shielding Plax
Enchantment — Aura
( can be paid with either 
or 
.)
Enchant creature
When this Aura enters, draw a card.
Enchanted creature can't be the target of spells or abilities your opponents control.
Brian Hagan
Rulings
If the target creature is an illegal target by the time Shielding Plax tries to resolve, the Aura spell doesn't resolve. It won't enter the battlefield, so its enters-the-battlefield ability won't trigger.
(wotc • 2018-12-07)
This is not the same as hexproof. If, for example, you target one of your opponent's creatures, your opponents won't be able to target their own creature with spells or abilities.
(wotc • 2013-04-15)
Rulings
If the target creature is an illegal target by the time Shielding Plax tries to resolve, the Aura spell doesn't resolve. It won't enter the battlefield, so its enters-the-battlefield ability won't trigger.
(wotc • 2018-12-07)
This is not the same as hexproof. If, for example, you target one of your opponent's creatures, your opponents won't be able to target their own creature with spells or abilities.
(wotc • 2013-04-15)