I had this error installing a legacy webpack config with npm:

 error node-sass@7.0.1 postinstall: `node scripts/build.js

The problem

The issue here can be that you have numerous dependencies that after a while conflict with node-sass. For example when adding webpack 

Solution

node-sass is deprecated. Instead, use sass.

You can uninstall the old one and install the new one

npm uninstall node-sass
npm i sass
npm i sass-loader