I am logging what appears to be a bug/software defect with Field Maps and Feature Templates.
Problem: I was unable to prepare a map for offline use when the Web Map referenced an empty Feature Layer with no records - and thus no Feature Template.
You will get an immediate failure message when preparing the managed map areas or manually downloading the map areas from the phone.
Solution: Create a new feature (and thus the Feature Template) before preparing your Field Map app for the field. You can delete the point after.
Environment:
AGOL
Field Maps for iOS (App Version 21.1, Build 396)
It would be great if this was better documented in online tutorials/help. Thank you
