GeneralUser

I recently had this same problem myself. I know how frustrating it can be, so I'm posting this so if anyone else has this problem they will know how I solved it. To fix this I had to start a whole new game, and these are the only mods I had activated: Skyrim.esmUpdate.esmUnofficial Skyrim Patch.espDawnguard.esmUnofficial Dawnguard Patch.espHearthFires.esmUnofficial Hearthfire Patch.espDragonborn.esmUnofficial Dragonborn Patch.espRaceCompatibility.esmSkyUI.espAchieveThat.espRaceMenu.espThe Ningheim.espRaceMenuPlugin.espSkyRe_Main.espRaceCompatibilityUSKPOverride.espCivil War Overhaul.esp I wish I could say exactly which mod or combination of mods causes this problem, but starting a new game with this relatively basic setup solved it for me.

I don't believe the knowledge of building new securitrons is lost. At least not to the Courier. At Big MT there are clearly facilities that can manufacture advanced robots (Dr. Mobius' robo-scorpions were built on site). There is even a reasonably well preserved facility specifically dedicated to deconstructing and analyzing securitrons. It does not take much imagination to think that those facilities could be salvaged and/or retooled to be able to manufacture new (or even upgraded/improved) securitrons.