Q:

Grouping error with prevgroupva() function

I am trying to create a month-over-month quantity difference report, but when I enter something like the following:
PREVGROUPVAL(Quantity__c:SUM, Report_Date__c)

I get the following error: 
You must select a grouping context to use any report summary function

I've tried various iterations of summary fields and grouping fields and can't get it to work using any data.  Thoughts?

A:

If you are doing this in lightning, I suggest editing the report in Classic.  For the PREVGROUPVAL function you need to choose a grouping level, which it seems like you can't do in Lightning.

