Error: Please install Git client from https://git-scm.com/downloads
Git is installed via initial Setup (Windows Atom)
Do you see it in PIO IDE Terminal?
git --version
I get this error, too, after fresh Atom/PlatformIO install on Windows 10.
git --version output:
git version 2.15.1.windows.2
Screenshot, below, shows error during build (in terminal window) as well as popup error boxes from IDE:

(And I restarted Atom after Git install (including multiple reinstall attempts))
Please check that git is in your system PATH. Maybe, OS reboot will help.