Calculated Item greyed out

[nmss18]

Hello,
I am trying to create a calculated item from a pivot table but the calculated item option is greyed out.(Interestingly the calculated field is not greyed out).
The source data is - as far as I can tell - is manually created and not from an external source eg OLAP cube.
Does anyone have any ideas why this is happening and how I can 'ungrey' it?
Thanks,
nmss

 

[nmss18]

I found the issue here. You have to select (ie place your cursor on) specifically on a field item and the calculated field becomes activated.