Hi @ventodimare
Try solution D first may be true.
That bolt looks as if it is meant to pass through that top bracket and square steel and out the other side where you said you have a lock nut ready for it.
A So If the above is true then tap it down with a hammer gently as there is no thread at the top in those bits of metal. If the fit is tight it could be becasue the hole of the top bracket is missaligned slightly with the hole in the square tube. So once the long bolt gets to the bottom of the square tube you will need to try wiggle/move the top bracket left or right until the bottom hole aligns too and the bolt can be tapped through. Now a spanner at the top and a spanner on the bottom is needed for tightning a lock nut.
B In a rare case the holes might not be drilled to size properly test this by moving the top bracket up and test the bolt through hole same for the square tube separatly.
C Wrong size bolt and nut supplied?
D On another note that square tubing looks like a furnature horizontal foot with a rubber boot on it. If That is so then the bolt should be inserted from underneath going upwards so the end of the bolt tail dosnt dig into the floor potentially. It is even possible manufactures designed it on purpose to have larger hole at bottom to take the bolt body diameter and the hole at the top is slightly smaller that allows the thread to protrude through both tubing and top bracket followed by the captive nut. So it stops incorrect bolt orientation. Just speculation.
