安裝的webpack版本是4.9.1，webpack-cli版本號是2.1.4。執行webpack命令提示：
One CLI for webpack must be installed. These are recommended choices, delivered as separate packages:

webpack-cli (https://github.com/webpack/we…
The original webpack full-featured CLI.
webpack-command (https://github.com/webpack-co…
A lightweight, opinionated webpack CLI.
We will use “npm” to install the CLI via “npm install -D”.
Which one do you like to install (webpack-cli/webpack-command):

解決：
需要安裝一個webpack-cli
npm install webpack-cli -g
就可以了

或者 npm install webpack-command -g （這個就沒有測試了）