Accepted answer

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 ( 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)

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