It sounds like you already have things installed.
So, download the plugin repo, put it inside of your plugin folder, activate the plugin and it works. Right?
Then when you open up the folder for that plugin in your code editor or conmand line and run npm install nothing happens except those two warnings? Are you in the folder that has a package.json file there? It will need that for npm install to work so it you’re in the wrong directory that may be the problem too.