How to open NM box knockouts?
The knockouts on plastic boxes are not designed to be removed completely. Rather, they are kinda like trap doors and some pressure against the NM cable helps keep it from slipping back out of the box. In your case, if you have old fabric covered cable, you really should be using boxes with a 1/2" screw type cable clamp. If you insist on using the plastic boxes, then you will have to use a screwdriver and hammer to knock the square door in first, then grab the flap with a pair of linesmen pliers and twist it until it separates from the box.  I guarantee that is going to leave a ragged edge that may still abrade your cable. These plastic boxes were never designed for old degrading cable.  BTW, I was the factory rep for Carlon Inc, manufactures of plastic boxes, many years ago, so I have a bit of background.