[Vuejs]-Cannot add Sass to Vue project in Webpack 3

The version of Webpack in your project (3.x) is old and incompatible with the newest versions of node-sass and sass-loader (5.x and 10.x, respectively).

For Sass in Webpack 3, install node-sass 4.x and sass-loader 7.x:

npm i -D node-sass@^4 sass-loader@^7