Mac OS 8.6 is compatible with MacOS.app with Mac OS X Server 1.0-2, which is Mac OS X Server with the Mac OS X Server Update 1.0-2 applied. This can be verified by checking the console window in Workspace. The first line should be "Software Version Mac OS X Server+Mac OS X Server 1.0-2 Patch (Hera1O9+Loki2G1)."
Mac OS 8.6 is not compatible with previous versions of MacOS.app.