ERROR in No configuration provided for /resources/css/app.css

Question

I'm a bit new to webpack and TailwindCSS. The stack we are using here is Laravel with Tailwind and predefined template.

I'm trying to start NPM but get the following error:

ERROR in No configuration provided for ....../resources/css/app.css

My webpack.mix.js has

mix.js('resources/js/main.js', 'public/js')
    .postCss('resources/css/app.css', 'public/css', [
        require("tailwindcss"),
    ]);

And webpack.config.js has:

const config = require('./site.config');
const loaders = require('./webpack.loaders');
const plugins = require('./webpack.plugins');

module.exports = {
  context: path.join(config.root, config.paths.src),
  entry: [
    path.join(config.root, config.paths.src, 'js/main.js'),
    path.join(config.root, config.paths.src, 'css/style.scss'),
  ],
  output: {
    path: path.join(config.root, config.paths.dist),
    publicPath: '',
    filename: '[name].[hash].js',
  },
  mode: ['production', 'development'].includes(config.env)
    ? config.env
    : 'development',
  devtool: config.env === 'production'
    ? 'hidden-source-map'
    : 'cheap-eval-source-map',
  devServer: {
    contentBase: path.join(config.root, config.paths.src),
    watchContentBase: true,
    hot: true,
    open: true,
    host: config.dev_host,
  },
  module: {
    rules: loaders,
  },
  stats: 'errors-only',
  plugins,
};

There is no where a reference of app.css... What am I doing wrong.