You may target a creature that is already tapped with Solitary Sanctuary's first ability. If the target creature is already tapped as it resolves, you will still put a stun counter on it.

Solitary Sanctuary's last ability will trigger only when an effect instructs you to tap an opponent's creature. It won't trigger if a spell or ability you control instructs an opponent to tap a creature they control. For example, if you control Tangle Wire and an opponent taps an untapped creature they control as part of the resolution of Tangle Wire's triggered ability, Solitary Sanctuary's ability won't trigger.