title: Vue2 - Error: PostCSS received undefined instead of CSS string
body: <p>i have this problem when trying to compile a Vue2 project</p>
  <pre><code>Syntax Error: Error: PostCSS received undefined instead of CSS string
   @ ./src/assets/sass/main.scss 4:14-233 14:3-18:5 15:22-241
   @ ./src/main.js
   @ multi (webpack)-dev-server/client?http://192.168.1.202:8080&amp;sockPath=/sockjs-node (webpack)/hot/dev-server.js ./src/main.js
  </code></pre>
  <p>I have tried to rebuild the sass-loader with no success.</p>
  <p>My node version is v16.6.1
  My NPM version is 7.21.0</p>
  <p>Webpack is 5.1.2 and the sass-loader version is 10.2.0</p>
  <p>This is a VUE2 project and all started when i started a new Vue3 project and i had to upgrade the vue/cli but i cannot understand how to go back locally (if this is the problem)</p>