Fry said in -lan starts game instead of server:
.\bin\plutonium-bootstrapper-win32.exe iw5mp "C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 3" -lan
@echo off
.\bin\plutonium-bootstrapper-win32.exe iw5mp "C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 3" +name "YEET" -lan
Just change YEET to whatever name you want. That will fix the error.