Technical Blog Post
Calendar field becomes required if a value existed before in this field
On Labor record the Calendar field (labor.calnum) becomes required after it is entered once.
After the Primary Calendar value on the Person (People) record has been added it becomes a required field and it is not possible to just remove the value and save the record or you will get the error above.
In order to remove the Primary Calendar value you must also remove the Primary Shift value in order to be able to save the record.
This will also remove the value in the Calendar Organization as this is the Organization for the Primary Calendar.
Please note that this has to be done in the Person application as this will not work if tried in the Labor application.