Pivot table Calculated item error

[Crazyjoe06]

I am trying to make a calculation based on a sub category but keep getting an error.

I have rows as different departments, my column as severity C and D, and another row for day/night. I am trying to calculate C/(C+D). When I click calculated item I get the following error: if one or more fields in a pivot table have calculated items, no fields can be used in the data area two or more times. I have looked everywhere on how to solve this error.