Unable to create Git index.lock File Exists

fatal: Unable to create '/Users/username/Sites/testsite/.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.

Just delete the index.lock file

rm /Users/username/Sites/testsite/.git/index.lock
git pull