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.