Git Error : 'upstream' does not appear to be a git repository

The article you link to (though it may have changed since you asked) starts with a reference to https://help.github.com/articles/configuring-a-remote-for-a-fork/. If you complete this you will have a new remote repository named upstream pointing to the original repository which you forked, and git fetch upstream will work.

So there's a few things to clarify:

• Upstream is the conceptual name of the remote repository (or repositories) that exist. Most projects only have one upstream repository.

• The name of the upstream repository can vary from project to project, but by convention is origin.

That said, I'm willing to bet that you got the name of your upstream repo and the upstream repo concept confused, and you should be executing git fetch origin. Verify with git remote; use the appropriate name from that list.

However, if you only have one upstream repo, performing git fetch will accomplish the same thing.