Node and npm should be installed via the package manager, its better this way. Follow this guide: Installing-Node.js-via-package-manager
If you already have node and npm installed, check that it is installed correctly:
apt-cache search ^nodejs$
If that does not return "nodejs - Node.js ..." you might want to consider deleting the current install and re-installing. Feel free to see me (Matt) for help installing on Ubuntu or other Linux distros.