It looks like the error is telling you that you don’t have a dev_server: config in your webpacker.yml file.
When you run webpack-dev-server in a Rails project, it looks for the dev_server: configuration in webpacker.yml. The keys/values map to webpacker-dev-server settings.
Take a look at your webpacker.yml file and confirm you have a dev_server: config present. Here is an example of a default (I don’t believe I made any changes) config:
# Reference: https://webpack.js.org/configuration/dev-server/
# Inline should be set to true if using HMR
Also, you’ll want to make sure you are not running webpack-dev-server in production. It is a development tool to allow hot reloading, not something that should be running in a production app. Cheers!