Keep awake while charging should be available on the S6. However, remember that the display is AMOLED. What you intend to do by using that feature is a good way to burn the screen.
A better option is to install Google News and Weather, set it up, then go to settings>display>screensaver, and choose that. This will show clock, news, and headlines on the screen while the phone is charging, but it will also move around the display like the AOD clock on the S7, making it much less likely to cause burn in.