Did you install the prerequisite mods that Buffout needs? address library and xse plugin preloader? and have F4SE running? (you can tell its running if you get a little terminal window pop up when you launch the game, but before the game loads. If you dont see this and you have F4SE installed, switch to a GE Proton. I use GE proton 9-5)
Buffout fixed it for me, but i heard another alternative was to lower your graphics quality until you meet the guy after the train (tryin to be vague for spoilers), then you can crank them back up.