"Fatal error: Unable to find local grunt." when running "grunt" command

All is explained quite nicely on gruntjs.com.

Note that installing grunt-cli does not install the grunt task runner! The job of the grunt CLI is simple: run the version of grunt which has been installed next to a Gruntfile. This allows multiple versions of grunt to be installed on the same machine simultaneously.

So in your project folder, you will need to install (preferably) the latest grunt version:

npm install grunt --save-dev

Option --save-dev will add grunt as a dev-dependency to your package.json. This makes it easy to reinstall dependencies.

You have to install grunt in your project folder

1. create your package.json

   $ npm init
   

2. install grunt for this project, this will be installed under node_modules/. --save-dev will add this module to devDependency in your package.json

   $ npm install grunt --save-dev
   

3. then create gruntfile.js and run

   $ grunt 
   

I think you have to add grunt to your package.json file. See this link.