The git flow is not installed in your machine. Just run this command,
macOS:
$ brew install git-flow-avh
Linux:
$ apt-get install git-flow
Windows:
$ wget -q -O - --no-check-certificate https://raw.github.com/petervanderdoes/gitflow-avh/develop/contrib/gitflow-installer.sh install stable | bash
Source git-flow cheatsheet by Daniel Kummer
After successful installation run git flow init in your project and then follow the instructions to configure the git flow.