Hello Lord Wack. KSE can only change the appearance of T3-M4. Think of it like a disguise. It's really still T3-M4 underneath. To actually replace T3-M4 with Atris would require a recruitment mod. Darth333 outlines this in a good tutorial
here.
@Greatness7, do you have savenfo.res in that directory mentioned in the error popup?