The plugin requires that you have gitflow installed, specifically the AVH edition. This is because the Vanilla Git Flow hasn't been maintained in years
How to check Git Flow version
run git flow version
If it says 0.4.1
then you have the wrong version. You will have to uninstall it and then refer to AVH edition docs for installation.
How to uninstall wrong version on OSX
If installed via brew
then run brew uninstall git-flow
Mac/Linux users:
If you're running into issues like getting
Gitflow is not installed
or
git: 'flow' is not a git command. See 'git --help'.
Please be sure to check out this thread