As he says on his website:
THIS REQUIRES RPG MAKER MV 1.3.0 OR ABOVE TO WORK! Please check your rpg_core.js file to make sure it says 1.3.0 or above. If it isn’t updated, visit this thread
for the update.
You might want to make sure that you have the right version.
Also he says:
There’s a couple of things you must do in order to get this plugin working.
1. You must have the Doodads.json inside your project’s ‘data’ folder.
2. You must have a ‘doodads’ folder inside of your project’s ‘img’ folder (unless you named it something else in the plugin parameters).
3. You must have your doodads within this folder.
You can find the above resources in the links above!
Close to the top of the page he has those resources.
If that still does not work, recopy the script.
(This is the page I am looking at: "http://yanfly.moe/2016/09/17/yep-116-grid-free-doodads-rpg-maker-mv/
If this helped, please respond so I know if I helped!