Try:
npm install -g express-generator@3
That solved problem for me.
Edit: for version 4
npm install express-generator -g
Description:
express is the package for dependency of express js.
express-generator is the package for enabeling express command and create a sample project, etc.
Assuming that this is kept separate for the decoupling of project dependency with cli tool of express.
Another SO ref: https://stackoverflow.com/a/41311733/1666582