Syntax Error: Error: PostCSS received undefined instead of CSS string

Syntax Error: Error: PostCSS received undefined instead of CSS string
Module Error (from ./node_modules/sass-loader/dist/cjs.js):
Cannot find module 'sass'
Module build failed (from ./node_modules/postcss-loader/dist/cjs.js):
Error: PostCSS received undefined instead of CSS string

遇到以上问题只需要重新安装：

npm install -D node-sass

npm install -D sass-loader