Getting an error when trying to add another column in OTBI Rep for a DFF

Summary:

There is a custom field (DFF) on the Customer Sites (Receivables) which we are trying to add to an existing OTBI Report.

After adding it to the report, getting following error in Results..! (Please see the screenshot under Content)

What could be possible cause & how to rectify it to get that column added..!?

One more thing, on trying to add that DFF column while taking a couple of columns from "Receivables-Customer real time" & "CRM - CRM Activity Real Time" Subject areas, the value for that DFF is coming as "__NOT_APPLICABLE__". So when does OTBI return such a value???