Ensure that the View | Toad Options | Source Control | Default VCS | Settings | GIT Client is pointing to 'GIT.exe'. If the GIT client is not mentioned in PATH, then ensure that the full location is stated e.g "C:\Program Files (x86)\Git\bin\git.exe" or C:\Program Files (x86)\Git\cmd\git.exe
If it's already pointed to 'GIT.exe', but it's still not working, re-enter the 'full' path i.e. navigate to the GIT install directory an put in the 'full' path.
