Why am I getting the message, "fatal: This operation must be run in a work tree?"

This should solve it:

git config --unset core.bare

The direct reason for the error is that yes, it's impossible to use git-add with a bare repository. A bare repository, by definition, has no work tree. git-add takes files from the work tree and adds them to the index, in preparation for committing.

You may need to put a bit of thought into your setup here, though. GIT_DIR is the repository directory used for all git commands. Are you really trying to create a single repository for everything you track, maybe things all over your system? A git repository by nature tracks the contents of a single directory. You'll need to set GIT_WORK_TREE to a path containing everything you want to track, and then you'll need a .gitignore to block out everything you're not interested in tracking.

Maybe you're trying to create a repository which will track just c:\www? Then you should put it in c:\www (don't set GIT_DIR). This is the normal usage of git, with the repository in the .git directory of the top-level directory of your "module".

Unless you have a really good reason, I'd recommend sticking with the way git likes to work. If you have several things to track, you probably want several repositories!

Also, you are probably inside the .git subfolder, move up one folder to your project root.

Just in case what happened to me is happening to somebody else, I need to say this:
I was in my .git directory within my project when I was getting this error.
I searched and scoured for answers, but nothing worked.
All I had to do was get back to the right directory ( cd .. ).
It was kind of a face-palm moment for me.
In case there's anyone else out there as silly as me, I hope you found this answer helpful.