即使我已经安装了webpack，npm也会说“未安装webpack，请安装webpack开始捆绑”

Question

我正在尝试按照here上的说明制作一个包含CSS模块和Webpack的网页。在运行npm start之前，我还不错，这就是它给我的错误：

> css-modules@1.0.0 start /Users/username/Desktop/css-modules
> webpack && open index.html


webpack not installed

Install webpack to start bundling: 
  $ npm install --save-dev webpack

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! css-modules@1.0.0 start: `webpack && open index.html`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the css-modules@1.0.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/username/.npm/_logs/2020-05-06T04_09_47_528Z-debug.log

…即使我已经安装了webpack。

我该怎么办？