I am new to
fatal: Unable to create '/.git/index.lock': File exists.
If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.
Assuming you're not doing anything with git at the moment (i.e., not doing a push or pull or running a git script in the repository, for any reason), you could just remove the lock file manually and try again.
Also, git expects a "commit message" describing your changes. Assuming you don't want an editor to open, you can provide an inline message using the
git commit -am "Changed this, that, and the other thing"