Switch in 1 of your opponent’s Benched Pokémon to the Active Spot. If you do, switch your Active Pokémon with 1 of your Benched Pokémon.
The first sentence must be able to be fulfilled in order to play Prime Catcher. Your Opponent must have a Benched Pokémon to let you play Prime Catcher; if your Opponent does not have a Bench, you cannot play Prime Catcher. You need a valid target (a Pokémon on your Opponent's Bench) to play Prime Catcher.
If you have a Bench, then after switching in one of your Opponent's Pokémon, then you must switch out your Active. If you do not have a Bench, then your Active stays.