If Shifting Shadow can't become attached to the creature you put onto the battlefield, most likely because that creature has protection from red, it becomes attached to nothing. It will be put into its owner's graveyard as a state-based action.
Shifting Shadow
If you don't reveal a creature card this way, you'll just reveal and randomize your library and Shifting Shadow becomes attached to nothing. It will be put into its owner's graveyard as a state-based action.
If the enchanted creature can't be destroyed, most likely because that creature has indestructible, you'll still get a new creature and attach Shifting Shadow to the new creature.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Christopher Burdett
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
