I'm trying to make it so an NPC doesn't get knocked back when hit in combat. I've tried NPC.setmotiontype(NPC.Motion_Keyframed)
but it doesn't seem to work on actors?

You want to eliminate stagger or have the NPC stand still completely and do nothing?

If the former, I'd recommend creating a new perk (you can use Tower of Strength as a reference) that eliminates all stagger chance.

If the latter, I think you'll need to do something with AI packages, but I can't really help you there.