I think maybe you should take it to a gunsmith, or try to find someone near you who is more knowledgeable.

EDIT:
I agree with what Robin posted, #3. There’s no trick. The nut is installed from the breech, not the muzzle. Sometimes the threads are too tight due to manufacturing tolerances. In which case light sandpaper can be used on the barrel threads or threads in the nut to take the edges off. Maybe 400grit. If it still won’t work there is an issue that requires a trained eye.