Hi there!
First make sure you are using the core plugin, EliMZ_Book.js somewhere above the Class Curve Plugin.
If the error still happens, I believe the problem is on the plugin parameter configuration. There is something that is not properly filled or something that must be filled, but it is empty. You can try show me screenshots of your Class Curve plugin parameter or upload that project replicating the error and send to me so I can take a look.