How to configure webpack correctly?

I try to complete this tutorial, but I have some trouble with configure webpack i think. I was making tutorial step by step but it doesn't work..

ERROR] Failed to execute goal com.github.eirslett:frontend-maven-plugin:1.10.3:webpack (webpack build) on project shop: Failed to run task: 'webpack.js ' failed. org.apache.commons.exec.ExecuteException: Process exited with an error: 2

error looks like:

[INFO] ERROR in ./src/main/js/app.js
[INFO] Module not found: Error: Can't resolve './client' in '/home/kk/IdeaProjects/e-commerce/src/main/js'
[INFO]  @ ./src/main/js/app.js 29:13-32
[INFO] 
[INFO] ERROR in ./src/main/js/app.js
[INFO] Module not found: Error: Can't resolve 'react' in '/home/kk/IdeaProjects/e-commerce/src/main/js'
[INFO]  @ ./src/main/js/app.js 25:12-28
[INFO] 
[INFO] ERROR in ./src/main/js/app.js
[INFO] Module not found: Error: Can't resolve 'react-dom' in '/home/kk/IdeaProjects/e-commerce/src/main/js'
[INFO]  @ ./src/main/js/app.js 27:15-35

webpack.config.js:

var path = require('path');

module.exports = {
    entry: './src/main/js/app.js',
    devtool: 'sourcemaps',
    cache: true,
    mode: 'development',
    output: {
        path: __dirname,
        filename: './src/main/resources/static/built/bundle.js'
    },
    module: {
        rules: [
            {
                test: path.join(__dirname, '.'),
                exclude: /(node_modules)/,
                use: [{
                    loader: 'babel-loader',
                    options: {
                        presets: ["@babel/preset-env", "@babel/preset-react"]
                    }
                }]
            }
        ]
    }

};

app.js:

'use strict';

const React = require('react');
const ReactDOM = require('react-dom');
const client = require('./client');

class App extends React.Component {...}
class EmployeeList extends React.Component{...}
class Employee extends React.Component{...}
ReactDOM.render(<App/>, document.getElementById('react'))