I was following along on the ESRI blog posts about Arcade elements from early last year: https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/part-2-introducing-arcade-pop-up-con... and was trying to figure out how to configure a Fields List or Text added from "Add Content" to a popup...
And then I realized that all the examples were using "Arcade" from that Add Content button which wasn't available on our Enterprise Portal.
What version of Enterprise was that made available in? I'm currently on 10.9.1.
You can find out the Arcade version your Enterprise uses here. Enterprise server 10.9.1 uses Arcade 1.15 while Enterprise portal 10.9.1 uses 1.13