Hi, you will need to use another plugin that can add custom battle commands, such as [CGMZ] Battle Command Window though any plugin that can add custom commands should work. In your custom command, put the JS:
this.showCGMZEncyclopedia(true); // change to false if from actor command
I am not sure why you are getting that error, are you sure all of your plugins are up to date? Can you show a screenshot of the full error trace from the dev tools console?