The error message is telling you exactly what you need to do to fix the issue.
git config --global user.email "firstname.lastname@example.org"
git config --global user.name "Your Name"
Obviously you should replace the values inside the quotes with your own email and name.
EDIT: You can also use the SourceTree Options dialog:
Hello! My name is Mark Askew and I am a Premier Support Engineer for products Bitbucket Server/Data Center, Fisheye & Crucible. Today, I want to bring the discussion that Jennifer, Matt, and ...
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