Since MSnap doesn't works on any firmware for the motoV6maxx I will show the way to take screenshots on your phone. It works on any firmware version.

You need to edit your seem 005b_0001 of your phone. Once you downloaded the seem from your Maxx do this edit, I prefer the XVI32 to do my edits.

Seem 005b_0001
Offset 19
Change the value to FF

Save it and upload it to your phone, and then press the VideoCall button. Your screenshot will appear on your picture folder, if the picture is not available, restart your phone and you'll see the image.

Here is my shot:

Also here is my edited seem with this mod made.
Seem 005b_0001

Before made this edit or upload my seem, make your own backup.
Enjoy.