Getting Error from webpack-cli: “TypeError: merge is not a function” in webpack config

You are importing merge incorrectly. Try it like this:

const { merge } = require('webpack-merge');

UPD:
Based on the following changelog, starting with webpack-merge 5.0.3 and higher, you should use the code I provided above. If the version is lower than 5.0.3, then you need to use:

const merge = require('webpack-merge');