Hello, sadly since this is a plugin you cannot add it to a vanilla Singleplayer world. But you can turn your world into a server if you wanted!
Find out how to make a Minecraft server:
https://www.youtube.com/watch?v=Le-TaZNzftg (use spigot 1.16.4)
To add your world to your Spigot 1.16.4 server, you must first find your Minecraft saves folder, in there you will see a folder with the name of your world. You can change the name of the folder to "world" and put it in your server files folder.
Make sure that the name of your world folder is "world".
NOTE: Hosting a Minecraft 1.16.4 spigot server will cost at least 1 or 2GB of ram.