webpack-merge has updated it's api in version 5.x.x. If following along in the course, the npm install would download version 5.x.x, where instead of using default require of webpackMerge it has to be updated to name import merge.

Change const webpackMerge = require("webpack-merge");

To const { merge } = require("webpack-merge");

To keep it similar to how Sean does it in the video, use version 4.x.x