That means the wellnut broke loose. You can try using a prybar between the bottom of the frame and the plate, prying down hard as you try to spin the bolt out. This works part of the time. Otherwise your gonna have to cut it off unfortunately.
There called nutserts or blind nuts. After you remove me it you drill a hole for the new nutsert. Slide the new nutsert in and tighten it down following the directions. The nut is inside the frame and crushes down on it to stay in place. Fairly easy to do. I had to to 4 out of 6 on mine. Hope this helps
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