I am using latest office 365 microsoft excel. I have created a pivot table with multiple tables. I wish to create and separate column for some calculation based on pivot table date. After researching I found that I can use calculated field option in Calculations tab. But the option gets greyed out when I use power pivot or multiple tables for one pivot table.
The option is greyed out because Calculated field translate in either Calculated column or Measure in Power Pivot. In other words what you get is expected
Your various Tables have same/similar content, then you can combine them with Get & Transform - aka Power Query - as a single Table that becomes the Source of your Pivot Table where you'll be able to add Calculated fields
You create Calculated column(s), ideally Measure(s) in Power Pivot (DAX reference)