Git Bash Command on Windows, yarn command not found

You probably just did not install yarn yet. Have a look at the yarn installation for Windows here:

https://yarnpkg.com/lang/en/docs/install/

The official Yarn setup does not tell you you need to restart your computer, but you actually have to before you can use it from your shell. So, install it from

https://yarnpkg.com/lang/en/docs/install/#windows-stable

then restart your PC and you should be able to run yarn from your console.

If you are using Mintty (Bash) then you simply do npm install -g yarn