My project is on the raspberry pi. I checked out the docs link you posted, the problem i was having was that the url provided to download node js on the docs was not compatible with the armh chip of the raspberry pi. It returned this error so curl -sL "https://deb.nodesource.com/setup_6.x" | sudo -E bash - returned this error:
You appear to be running on ARMv6 hardware. Unfortunately this is not currently supported by the NodeSource Linux distributions. Please use the ‘linux-armv6l’ binary tarballs available directly from nodejs.org for Node.js 4 and later.
I solved that by finding the the matching version for my pi and i installing as i type this. Will post back if i run into more complications.