Output filename not configured.

Webpack throws this error when there is a typo somewhere, or it could not find the config file. Check the following things to resolve the issue :

webpack.config.js named correctly without any typo (sometimes people forget to add .js extension)
webpack.config.js is in the root directory of the project
webpack.config.js has no typos
Export the JSON object from webpack.config.js
In terminal navigate to the directory where webpack.config.js then run the webpack command.

I hope one of those checks resolves your issue. If you would like to see more posts on common errors with webpack, let us know via your comments !