Critical Error when starting a new game. 6 years ago

After I bought and installed the newest DLC, I couldn't start a new game. It would always show a critical error message. I'm not using any mods, I verified game cache, uninstalled and reinstalled the game, made sure the app is allowed through my windows firewall, deleted registry.xml, running the game as an admin, and even uninstalled and reinstalled steam. Nothing is working. I see a few other posts of this similar issue with no actual answer given on how to fix this game breaking issue. Someone please help.