To unlock the Objection achievement in Little Nightmares 2 you'll need to throw the piece of cheese that can be found in the cell just after escaping from the mannequins in chapter 3 (the hospital chapter) into a hole that can be found in a secret room (the same room as one of the glitched remains). The secret room is located at the other end of the corridor where all of the mannequin arms try to grab you through the cell doors. Here is my own video guide that shows how to get the Objection achievement along with all of the other achievements Little Nightmares 2 has to offer. The guide for this achievement starts at around the 15:44 mark during the video.