How would one create a viewshed (using the 60 day free trail of the ArcGIS online)
maybe you can't use it in the trial since it requires elevated priviledges to use
Perform analysis—ArcGIS Online Help | ArcGIS
or maybe you have used them up
Create Viewshed—ArcGIS Online Help | ArcGIS
If I login to my trial AGOL Acc. I can see "Create Viewshed" under Perform Analysis > Find Locations
And with the AGOL trial you also get
200 ArcGIS Online service credits that can be used for data storage, premium data access or performing geocoding and analysis.
Retrieving data ...