sudo: docker-machine: command not found
You need to install Docker Machine first on your local machine. If you use Ubuntu, just use this snippet (Update the version from the Official Repository Releases if needed) :
$ curl -L https://github.com/docker/machine/releases/download/v0.16.0/docker-machine-`uname -s`-`uname -m` >/tmp/docker-machine &&chmod +x /tmp/docker-machine &&sudo cp /tmp/docker-machine /usr/local/bin/docker-machine
The accepted answer contains outdated install instructions!
Up to date instructions for Docker Machine can be found in the official documentation here. This includes instructions for MacOS, Linux (including Ubuntu) & Windows with Git BASH. Full documentation can be found here.
While @sdey0081's answer is more or less correct, running the posted commands will result in an outdated version of Docker Machine being installed. The version it installs is v0.13.0 while the current release at the time of writing is v0.15.0. You can find the available releases in the GitHub repo here.
On MacOS installing with brew is the simplest solution: brew install docker-machine