Solution:
You need to change the ownership of folder node_modules, because you use sudo npm install -g node-sass, so its ownership is set to root.
To change the ownership to the current user as group –
sudo chown -R root:$(whoami) /usr/local/lib/node_modules/
sudo chmod -R 775 /usr/local/lib/node_modules/
And never use sudo for an npm-related query.