Nuts tend to be a looser fit. It can be tbe same thread with a tighter fit. Check the reciever with a flashlight for any grit or blast media. Ss action are often sandblasted and the bead can lock up a barrel in a heartbeat. I would hose it with brake cleaner and oil it and try again.

If it still wont go take your old barrel and point the muzzle opposite from the new one and try to mesh the threads so they interlock. If tbey wont interlock than the barrel is threaded wrong. Not likely but some to check.

Flattening the tops of the threads wont hurt anything. Dont be scared. You can lightly file them and take scotchbrite to clean the up. Hose with brake cleaner and oil to try again