With versioning enabled, the Scheduled Activity property can only be enabled at activity creation. There is no option to convert an existing activity to a Scheduled activity.
If you plan to connect Events to an activity, ensure that you enable the Scheduled Activity property when the activity is created.
You can only connect Events to an activity while working on the Draft of that activity. Once Events are created, they are automatically connected to any Published version of the activity and made accessible to users.
When creating new events via the Events list, you will find that only Published activities are available for event assignment.