it's related to NPM v3's changed installation behavior.

You can resolve that issue by following methods:

Ensure dependencies described correctly on package.json
Just type npm install and hit Enter.
Check issue still exists. and If issue not resolved, continue these methods.
type npm cache clean and hit Enter
type npm install -g npm and hit Enter.
Retry npm install and hit Enter.

if issue not resolved even tried these methods, Please reply with your Node.js version, NPM version, Operating System, and package.json which is using.

ref link: https://github.com/visionmedia/debug/issues/261