The big part on the left is a BNC jack.
I haven’t opened the part but the reason I have had fill not work is because the area is made up of 2 paths. One on the outside of the line and one along the inside of the same line. To fix it you would select that element and then go to the path menu and select break apart (ctrl shift k). This will make two solid objects. One made using the outer path and one made of the inner path. Then you would delete one of them leaving a solid filled object.