Error: Cannot find module 'gulp-sass'
Solution 1:
Just do npm update and then npm install gulp-sass --save-dev in your root folder, and then when you run you shouldn't have any issues.
Solution 2:
Did you check this question?
May be possible solution is:
rm -rf node_modules/
npm install
Solution 3:
Edit your package.json. Change:
"gulp-sass": "^2.3.2"
to
"gulp-sass": "3.0.0"
Delete the node_modules folder and run npm install again.
Source: https://github.com/codecombat/codecombat/issues/4430#issuecomment-348927771