I had also facing this error. None of the above methods work for me. Please follow this. It's Works like a charm !!
For Installing node-sass in Ubuntu 16 via npm :-
You can install with npm 5.2.0 Version
If you are using nvm :-
nvm install 8.2.1 nvm use 8.2.1 npm install node-sass
If you are using npm separately then upgrade or downgrade npm version to 5.2.0
npm install node-sass