[Solved]-Webpack Entry module not found (by tutorial)-Reactjs

Looks like the path concatenation misses a slash, try

var HTMLWebpackPluginConfig = new HTMLWebpackPlugin({
        template: __dirname + '/app/index.html',
...

The probably better way would be, however, to use the path utility module (https://nodejs.org/api/path.html) like this:

const path = require('path')
...
template: path.join(__dirname, 'app', 'index.html')
...

This makes path concatenation less error-prone and is OS independent (backslash vs slash on windows vs *nix based os)