How to flash Stock Android 6.0.1 on a Samsung Galaxy S7 G930A (ATT)?

[Android Central Question]

I have a Samsung Galaxy S7 G930A (ATT) and I am trying to restore it to Android 6.0.1 from Android 8.0.0. I am having a really hard time finding a Stock ROM for my phone. I downloaded two of them from different websites. Loading them into Odin 3.13 goes just fine, but When I click on start, it gives an error message saying "FAIL! (Auth)" I have tried as much as I can but can't seem to figure this out. Anything Helps!

 

[Rukbat]

Samsung sets flags to determine the level of Android the phone updates to, and you can't go back before the flag. You might be able to go back as far as 7, but evidently not to 6. (The flags are actually fuses - updating blows one, so you can't "reset the flag".)