React-Cesium webpack config loader issue - reactjs

Question

I'm new to playing with Webpack configuration and attempting to incorporate React-Cesium into my project. I'm following along with the Github instructions for installing React-Cesium and running into this error:
Here is my webpack.config.js file after adding what the documentation told me to:
'use strict';
const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
module.exports = {
module: {
externals: {
cesium: "Cesium"
},
output: {
publicPath: "/"
// ...
},
alias: {
cesiumSource: "cesium",
cesium: "cesium/Cesium"
},
rules: [
{
test: /\.pug$/,
use: [
'pug-loader?self'
]
},
{
test: /\.css$/,
use: [
'style-loader',
'css-loader'
]
},
]
},
plugins: [
new UglifyJSPlugin()
new CopyPlugin([
{
from: `node_modules/cesium/Build/Cesium${prod ? "" : "Unminified"}`,
to: "cesium"
}
]),
new HtmlIncludeAssetsPlugin({
append: false,
assets: [
"cesium/Widgets/widgets.css",
"cesium/Cesium.js"
]
}),
new webpack.DefinePlugin({
"process.env": {
NODE_ENV: JSON.stringify(opts && prod ? "production" : "development"),
CESIUM_BASE_URL: JSON.stringify("/cesium")
}
})
}
],
};
The documentation is a little ambiguous and I'm not sure what's wrong. The documentation says to install npm i html-include-assets-plugin --save-dev. That command doesn't work, I get a 404 error when attempting to install it.
Is that what is causing the loader error? If not, then what is?