Based on this, I believe the Git repository was not initialized. Note that once the repository is created it should be initialized following the next steps:
echo "# Readme" >> README.md
git add README.md
git commit -m "first commit"
git remote add origin <git_url>
git push -u origin master
I think you received the above error because there's no Git initiated in the repository yet and no "ref"(HEAD) to use. Before you can create environments, you will need to commit and push elements from your database into the Git repository. The VDP server will automatically run 'git init' for the first push.
Hope this helps!