Why is the nut not engaging properly with the screw when fitting the handle of my cylinder mower? Why can't the handle of my cylinder mower be fitted correctly?

The screw length of the screw must be 160 mm.
Please note that the square collar of the screw can only be fitted on one side. If it is on the wrong side, the screw is too short.
If the screw is inserted on the wrong side or not inserted far enough into the square collar, the thread of the nut cannot engage.