You need to install it globally

npm install -g nodemon
# or if using yarn
yarn global add nodemon

And then it will be available on the path (I see now that you have tried this and it didn’t work, your path may be messed up)

If you want to use the locally installed version, rather than installing globally then you can create a script in your package.json

"scripts": {
    "serve": "nodemon server.js"
  },

and then use

npm run serve

optionally if using yarn

# without adding serve in package.json
yarn run nodemon server.js
# with serve script in package.json
yarn run serve

npm will then look in your local node_modules folder before looking for the command in your global modules