It seems as though I am unable to create a predominance map in ArcGIS Online when using a published map service using Smart Mapping. In the screenshot below I'm able to add only 3 variables and I only get the option to use the 'Relationship and Size' style. Using Arcade, I can create a predominance visualization with this rest service fine (from this ESRI Blog article). Also, if I use a hosted feature layer it seems to work as well. Just curious why it does not work with a published map service?
When we first implemented Predominance mapping, ArcGIS Server services didn't expose the necessary tech to draw layers this way. But now they do. So this feature sort of slipped through the cracks. Thanks for bringing it to our attention. We will address it at a future update.