I am trying to integrate my webpack project with sass-loader, but I am get this error:

 Module build failed: Error: Cannot find module 'postcss'

I already installed postcss and postcss-loader with command:

 npm install postcss postcss-loader --save-dev

After install, I continue to get the same error message. I test integrate my project only with css-loader and the same error occurs. I think postcss is a dependency of the css-loader.

In my webpack.config.js I use this configuration in my module.rules:

 {
    test: /\.scss$/,
    use: [{
          loader: "style-loader" // creates style nodes from JS strings
      }, {
          loader: "css-loader" // translates CSS into CommonJS
      }, {
          loader: "sass-loader" // compiles Sass to CSS
      }
    ]
  }

I installed all this loaders and node-sass, I don't know what I am doing wrong :(