There are two reasons that might prevent to add or edit a schedule
Reason 1:
If the schedule is PUBLISHED, the top left will show a button to unpublish, in red
When it’s unpublished, it will turn green to publish
Reason 2:
Employees who are marked as ‘NOT scheduled and can check in/out at any time’ will show on the schedule page, however they will not have the option to add a schedule (because this option does not allow for schedules).