The Samsung Galaxy S6 (SM-G920A) from AT&T is a notoriously difficult device to modify due to its locked bootloader. While Android 6.0.1 Marshmallow is the final major update for this classic, rooting it requires a specific set of files and a careful process.

Once modified, you will no longer receive official over-the-air updates from AT&T. on 6.0.1, your best chance is to check the XDA-Developers S6 Forum

You can uninstall or disable AT&T and Samsung bloatware without root using the Android Debug Bridge (ADB). This frees up storage and improves battery life without modifying the system partition.

If you absolutely need root, sell/trade the G920A and purchase a device with an unlockable bootloader.