PluginManager is a class that exists in core RPG Maker MZ.

That line is meant to load the plugin commands from the plugin so plugin calls know what to do.

Hi there Battle Nix!

First of all, I'm so sorry. Somehow I miss your post... =/ If you want any paid plugin from me, add me on discord: hakuenstudio

I will send you a free key.

Now, your problem was that you are not using EliMZ_Book. Or it was not somewhere above the Battle Results on the plugin manager. The RPG Maker MZ plugin manager you send you an alert on the bottom of the plugin manager window in case you are not using the core plugin.

(MyEnemy, thanks for trying to answer!)