There are 2 reasons that can prevent to add or edit a schedule.

Reason 1:

If the schedule has been posted, no changes can be made to it. The top right of any posted schedule will alert you if this is the case:

If the schedule has NOT been posted, you can make changes to the schedules and the top right will show:

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).