How do I get root on AT&T Galaxy S6 Marshmallow - downgrade? Please help!

I have been trying to flash previous firmware version in Odin with no success. I keep getting the error: sboot.bin.....NAND Write....FAIL

In the end I just want root access so if I can get it on my Marshmallow build(G920AUCS4CPF1) and help is appreciated.

If I need to downgrade first, help doing that would be great. Root help please!!! I need root back in my life.