[Solved]: git: 'lfs' is not a git command. See 'git --help'.

Debian:

$ curl -s https://packagecloud.io/install/repositories/github/git-lfs/script.deb.sh | sudo bash

$ sudo apt-get install git-lfs

macOS (Using Homebrew)

$ brew update

$ brew install git-lfs

Ubuntu

$ curl -s https://packagecloud.io/install/repositories/github/git-lfs/script.deb.sh | sudo bash

sudo apt-get install git-lfs

Windows

Use the Windows installer here: 

https://github.com/git-lfs/git-lfs/releases

After installing, you can install using:

git lfs install