Mine did the same thing. It was a compression/pressed-in nut that is squeezed on both sides of the metal bracket from the factory. if you over-torque it, it just spins. my 13 y/o daughter did the same thing. I had to jam a flat blade screw drive into the nut to hold it in place while working the bolt. I had to order another header bar from the place i bought it because bestop didnt/wouldnt replace just the bracket. then once i received it swapped out the parts and shipped it back. what a PITA. Hind sight would be to drill a hole in the bracket allowing access to a nut from the under side. but there i go again thinking simple.
Hope this helps.