Cannot find module 'webpack' (CLI v2.0.0-beta.15)

The error is that it appears they assume you have webpack as a global executable.

The general solution is to execute

npm install -g webpack

I prefer to keep my dependencies in my local project however so I would run

npm install webpack --save-dev --save-exact

This will install the webpack dependencies locally and not use a wishy washy version number. So you are guaranteed to get the same version next time you have to install.