the fitting is easily cross threaded.
inspect it good and run the flair nut on it before you thread the pipe thread in as a test.
If the old one checks out you should be good to go.
Im surprised the FLAPS didnt have it
you can check this number
...pic... But without knowing your set up it could be the wrong number.
I always use a flare nut wrench in the inverted nut and back up the brass fitting with another wrench when tightening.
