If you're trying to add a calculated item, you cannot do so because a field has been used more than once, either in the data ...

If you're trying to add a calculated item, you cannot do so because a field has been used more than once, either in the data area two or more times, or in the data area and another area at the same time.  Change the PivotTable report so that no field is used more than once, and then try the calculated item command again.