1. Have you selected correct type of Script task for Mac clients such as AppleScript?
2. Run this command line from Terminal on this Mac
--// To check what Task Server serves this Mac client for tasks
--// To re-register Client Task Agent plug-in on Mac client with Task Server
--// To check what plug-ins are installed on this Mac client and make sure that appropriate Task type should work on this client. For example if DS task is scheduled on this client bu there is no DS plug-in installed, then DS task will fails.
--// To check latest log file output from ULM agent on Mac client to identify problem of task execution failure(s)
- cat /opt/altiris/notification/nsagent/var/aex-client.log