I recently tried to enable auto shutdown on a virtual machine. I wanted to receive a notification before auto-shutdown. But the option was greyed out.
It would appear if send notification is set to yes, then the Save option is greyed out. Set Notification to No and you can save the settings > Set the time and Save.
Is it a feature or a bug, this was my work around.. After you have enabled and set a time. Now edit again and set notifications, add and email address and Save is available.