The error you are receiving is because the plugin doesn't know how to handle what you have told it to do. Specifically doesn't know how to handle the sprite it thinks it should use. Which is why you are getting the error. I had a similar issue when I first started using RPG Maker MV with YANFLY plugins and hope the below will help. A great many developers, myself included, use this plugin without issue. This is why I recommend double checking.
Most notably, be sure you have the correct information in the notes section for your "enemy"
As an example you should have something like this in the "Note" section of the enemy
<Sideview Battler: Ritz>
<Scale Sprite: 66%>
Be ABSOLUTELY sure you have the correct name, and case in the name. If it is upper case R in "Ritz", then "ritz" will fail.
I hope this is of help and I made a comment asking for some additional information so I may be able to help further, which I will revise this answer with when, and if needed.
Hope you get it sorted and don't be discouraged by some of the quirks to configure them. YANFLY plugins are usually very good for RPG Maker MV.