Hi Rose,
I contacted ESRI a couple weeks ago inquiring about this topic and was notified that Arcade functionality for List tab is only available on ArcGIS Online at this time.
Quote from the rep...
"I have done some digging to find out when this functionality might be coming to Enterprise and after discussing with my team I have found that the new functionality available in ArcGIS Online is not guaranteed for future releases in ArcGIS Enterprise. Unfortunately this means I cannot say for sure if and when you may see these Arcade options. However, there is an existing enhancement request which is currently marked as "Under Consideration" by development: ENH-000130995 - Allow embedded Arcade expressions in ArcGIS Dashboard widgets in ArcGIS Enterprise"
I am hoping they bring this functionality in Enterprise as I find it very useful. I hope this helps.
-Teejay