Octagon box wire access point

To use the knock out in the middle of the box you Need to use a lock nut type clamp.

You remove the lock ring from the clamp and pass the threaded part of the body through the knockout from the outside and then place the lock nut on the threads from the inside of the box And tighten.

Then you can thread your NM Cable (Romex) through the clamp and tighten down the clamp screws.

This article will explain it.

The knockout indicated on the top looks about the same as those on the sides -- same shape, same size, same distance to the corner, etc.

The clamp bar inside the box is held by a screw. Could you remove and re-install these so that the screw draws the clamp bar up against the side wall of the box, rather than against the top of the box as it is now? The pictures don't show the side wall of the box clearly so I can't see whether there's a hole already drilled for this in the side wall.

Junk the box. Get one that does not have interior cable clamps, and simply has multiple 1/2" knockouts. Use the cable clamp recommended by Alaska Man.

That is not the right box for you, since it does not do what you want.