When editing the scheduling calendar this error can occur. The scheduling calendar automatically follows the order of the cycle days defined in the Cycle. If Monday is an A day and we try to enter Tuesday as a B day we will see this error since Tuesday is already a B day. We should only be entering schedule calendar entries for holidays or for adjusting when a cycle day will fall.
Select a different cycle day or remove the scheduling calendar entry. To view the cycle days assigned to specific dates on the calendar, view the scheduling calendar by clicking the Calendar icon in the right hand corner of the scheduling calendar tab.