Trying to generate the RTL versions of CSS files in metronic framework with npm run build --rtl=true --demo1, I get following error:

[webpack-cli] Failed to load '/home/user/Documents/Metronic v8.0.32/html/theme/tools/webpack.config.js' config
[webpack-cli] ReferenceError: require is not defined
at file:///home/user/Documents/Metronic%20v8.0.32/html/theme/tools/webpack.config.js:1:17
at ModuleJob.run (internal/modules/esm/module_job.js:145:37)
at async Loader.import (internal/modules/esm/loader.js:182:24)
at async WebpackCLI.tryRequireThenImport (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:254:18)
at async loadConfigByPath (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:1710:19)
at async WebpackCLI.loadConfig (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:1830:30)
at async WebpackCLI.createCompiler (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:2185:18)
at async WebpackCLI.runWebpack (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:2321:16)
at async Command.<anonymous> (/home/user/Documents/Metronic v8.0.32/html/theme/tools/node_modules/webpack-cli/lib/webpack-cli.js:1058:13)
at async Promise.all (index 1)

webpack.config.js file is:

const webpack = require('webpack');
const path = require('path');
const fs = require('fs');
const del = require('del');
const glob = require('glob');
const CopyWebpackPlugin = require('copy-webpack-plugin');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const TerserJSPlugin = require('terser-webpack-plugin');
const CssMinimizerPlugin = require('css-minimizer-webpack-plugin');
const WebpackMessages = require('webpack-messages');
const ExcludeAssetsPlugin = require('webpack-exclude-assets-plugin');
const MergeIntoSingle = require('webpack-merge-and-include-globally/index');
const RtlCssPlugin = require('rtlcss-webpack-plugin');

.
.
....


module.exports = () => {
    return [mainConfig()];
};

How can I get rid of this error? Some other questions suggest some changes in module.exports section, but they don't solve the problem.