I'm having the same issue with the (Is DLC Installed) Plugin Command. When the DLC isn't Installed I get a "falsevariable is not defined" error and crashes the game. When the DLC is Installed the Variable doesn't hold the value of true.
I recommend you use the following plugin which has better support and updated features:
https://rabbitteaparty.itch.io/nekogakuen-steamworksapi
