Removing knockouts in old work metal boxes
A lot of handy boxes and old work &quot;gem&quot; boxes have knockouts that are made to knock from the outside in.  This picture of a Steel City handy box shows it clearly

If you have to remove these from the inside, there is a trick.  You can't pull the KO with a hammer.  But if you drive a small self drilling screw into the KO from the inside

then back it out a few turns, you can pull on that screw with pliers to remove the knockout.  If you can't get it with pliers, try a mini prybar