This PivotTable report field is grouped. You cannot add a calculated item to a grouped field. To ungroup the field, make ...

This PivotTable report field is grouped. You cannot add a calculated item to a grouped field.  To ungroup the field, make sure it is in the row or column area, select one of the items of the grouped field, click the Ungroup button in the Group group on the Options tab, and then insert the calculated item. After inserted, you can group the items of the field again.