When using the GitHub for Mac app (version 87), I received the following error:
git: \'credential-osxkeychain\' is not a git command. See \'git --help
this always happens with source tree app and the solution that you change the settings to use the system git not the embedded git with source-tree because source tree is using an older version or you can update the settings with the path to git executable and you should find it in /usr/bin/git or you can print which git and point your Source tree app to use it