Pivot table calculated column option greyed out

[LadyMagnus]

Hi, I am having trouble adding a calculated column to my pivot table.

I have 2 tables of data on spreadsheets in the workbook. Table 1: Participation by Program, Table 2: Census Population Data by Zip Code.

I am trying to find the % total participation for each zip code. To do this I created a relationship between the tables using the zip code. This relationship is working as intended, however, when I add a pivot table the option for calculated columns is greyed out. Any idea why that is the case?

Thanks!