git: 'flow' is not a git command. See 'git --help'
Remove git-flow and install it manually.
On Linux:
sudo apt-get remove git-flow && wget --no-check-certificate -q -O - https://raw.github.com/nvie/gitflow/develop/contrib/gitflow-installer.sh | sudo bash
On Windows
If you work with cygwin make sure util-linux and wget are installed. Then run this with cygwin:
wget -q -O - --no-check-certificate github.com/nvie/gitflow/raw/develop/contrib/gitflow-installer.sh | bash
For further information check the project page on github