webpack-dev-server Cannot find module 'webpack'

I had the same problem, but I resolved by installing additional webpack-cli

npm install --save-dev webpack-cli

npm install --save-dev webpack is not enough.

You also have to install the following:

npm install --save-dev webpack-dev-server 

And optionally, you can also install:

npm install --save-dev webpack-dev-middleware webpack-hot-middleware

Try the following:

Firstly, you need to install webpack-dev-server

npm install --save-dev webpack-dev-server

Then in your package.json, use this in your start script:

"start": "webpack serve --config webpack.config.js --open"

Note webpack.config.js should be replaced with the file name of your webpack config file.

Now run npm start

Hope it works!