Ok the very first thing to make sure of, is whether or not this is a Follower or a Companion. Companions should have the required keywords to wear PA.
Then, if they're a companion, make sure they either have a fusion core in their inventory, or there's a core in the PA.
Now, if they're just a Follower, they may not have the keyword entry that allows them to "ride" a suit of PA outside of a threat situation.
IF this is a custom follower added by a mod, you can open their file in Xedit https://www.nexusmod...lout4/mods/2737 to see if they have it, or add it in, if they don't.
If it's a vanilla settler, I really dunno if you want to mess with it.
But yeah, find the entry for the NPC in their esp file. They will be listed under Non Player Character and you click the + sign next to that to open that tree branch. Once opened, you just click on their name once.
Their info will open up in the right side panel. This stuff is at the lower end of their file, so I recommend that you just pull the scroll bar all the way to the bottom, and then start slowly moving your way back up the page.
Click on this to take a look at the pic to see what area of the file you're looking for. This is from Piper, btw.
You're looking for their Keywords section. Which I highlighted in Yellow. The line that I highlighted again, in Blue is the line that needs to be there.
If it's not there, they won't stay in PA.
But, on a mod file, you can just add it in. See that blank slot right there above the section that says "AnimArchetype....." ? Right click that empty spot, and then choose add. Then click on that new slot that gets added so that a cursor marker appears in it, and start to type in "p-Attach" and it should automatically take you to a drop box with that option to select it. Click on that option, then click another area of the file to exit the drop box.
Then hold down the CTRL key on your keyboard and tap the S key to save your changes.
Now again tho.... I warn you.... doing this to randomly generated characters like Raiders, Settlers, Gunners, etc is NOT a good idea. I don't know what can happen to them if you do this.
But hope this helps
Edited by StormWolf01, 19 February 2020 - 02:20 PM.