I have been having issues recently with getting Field Maps to work offline. I have the layer set to editable, sync is turned on, but when I am in the app on a mobile device it starts the download and then fails in a few seconds.
My current process is:
1. Open shapefile in ArcGIS Pro
2. Share shapefile as web layer
3. Open layer in ArcGIS Online, turn on editing/sync and save map using navigation basemap
4. Open Field Maps on device and try to add offline area, where it fails
In the troubleshooting log I see
"Job error -1 Service error code 500. Unable to create replica. Please check your parameters.
Details: Create replica internal error."
I have been able to work around this by adding the map data to a file that I previously used without issue in Field Maps, but recently that hasn't been working consistently. Most recently I had to create the layer 5 times before I could add an offline area without issue.