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
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.