A valid Gruntfile could not be found

I have followed this link to try to install a mean.js stack on an ubuntu server.

Regarding grunt, in /opt/mean, I have done

sudo npm install -g bower grunt-cli
sudo npm install grunt --save-dev
sudo npm install

However, grunt or grunt jshint inside the folder gives

A valid Gruntfile could not be found. Please see the getting started guide for more information on how to configure grunt: http://gruntjs.com/getting-started Fatal error: Unable to find Gruntfile.

Note that npm start works, and http://server_domain_or_IP:3000 can display the sample application.

Does anyone know how to fix the grunt error?

via SoftTimur