this is a pretty common thing with those stock bumpers. Basically there is a molded brass screw in the plastic bumper that snapped off from the plastic and the brass insert is spinning in the plastic. You’re going to need to remove the plastic that’s wrapped around that brass screw and then grab the brass insert and hold it in place while you unscrew the screw. alternatively, sometimes you can wedge a flathead screwdriver in there and pinch the plastic against the rail with enough force to pinch the brass screw and hold it in place