Compare your package.json and the dependencies in https://github.com/rails/webpacker/blob/master/package.json and avoid any duplicates. We don't want different versions of the same packages. We want the versions from rails/webpacker unless we specifically want to override them.
Search the rails/webpacker repo for anything you're not sure about in terms of package names.
run bin/webpacker and make sure there are zero errors
update webpack plugins and loaders to current or close to current
Make sure that your bin/webpacker and bin/webpacker match the latest on https://github.com/rails/webpacker/tree/master/lib/install/bin