The lever is at the end of the blue trial after you beat Aiko, where you are offered to pull all three.
Do not do that, only pull two levers randomly and then go back to the previous room.
Another lever as at the end after you beat Feng, the Minoraut girl will offer to pull it with you and you have to accept. The fact that you can only see a red button means this is already completed, so you have to finish the blue trials first.