error: unable to find utility "git", not a developer tool or in PATH

The best solution will be to download and install it - even that you assume it is already installed: https://git-scm.com/download/mac

I had this same problem -- I exited my bash session (closed the terminal) and started a new one and it fixed it.

This happened to me also with upgrading to High Sierra and XCode to 9. This sequence has fixed it.

xcode-select --install
sudo xcode-select -s /Library/Developer/CommandLineTools

Found this answer on https://davidwalsh.name/fix-git-high-sierra-upgrade (article and comments.)