Yesterday, I posted https://community.esri.com/thread/235847-mean-for-all-features-not-just-one-arcade and I was able to find the solution I was after, in ArcPro. But apparently, using $layer is not supported in Portal.
From yesterdays post: I have a polygon feature class representing Meals on Wheels data by month/zipcode: imagine a 'stacked' set of polygons. When I identify a given zipcode polygon, the pop up returns 12 values (one per month). That's fine but I'd like to show the mean of all 12 months.
I published the data to our internal portal but now when I try to use the same approach as in PRO, I get an error:
Some day I'll get a grasp on Arcade and what it can do, and where, and when...