Did you tried this tutorial?
http://www.samsungsfour.com/tutorials/how-to-root-samsung-galaxy-s6-for-at-t-sm-g920a-and-verizon.html

Yes I have this version though
Build: MMB29K.G920AUCS4CPG1says not supported yet ... my luck

Is your phone's bootloader unlocked? If not, please do it first and try the rooting steps again.

I followed the link to the bootloader apk unlocker and after install ran it it,crashes everything. I'm at a loss now to know what to do.

I actually Googled bootloader and found links for my version of phone etc. ctom.apk installed it and tried running it crashed every time. Having issues every time I try to root . Tried all the apks. Towel ping-pong etc

Unless and until your phone's bootloader is unlocked you will not be able to root it....

Like I said above I Googled how unlock the bootloader. Downloaded the correct Crom.apk according to the link. Samsung-sm-920a AT&T; build number is mmb29k.g920aucs4cpg1
Any more info you could send me to do this would be appreciated.
Garry

Hi,

I am very sorry to say this but as I already said bootloader unlocking is always a tedious process, that's why no tech blogs have a ready made solution for it.
I even doesn't know why carriers are making their customers life so hard.

You have to find the unlocking solution using Google, unfortunately there is no other option left.