Oracle Text

Enchant creature

When Claustrophobia enters the battlefield, tap enchanted creature.

Enchanted creature doesn't untap during its controller's untap step.

Card Rulings

6/22/2015 Claustrophobia can target and enchant a tapped or untapped creature.
6/22/2015 The enchanted creature can still be untapped in other ways. Claustrophobia will remain attached, and the creature will continue to not untap during its controller’s untap step.