I has publish service on Arcgis Enterpriser 10.7.1 and enable "Query" and "Sync" in capabilities (guide: Prepare data for offline use—ArcGIS Server | Documentation for ArcGIS Enterprise)
But I not see button "enable offline mode"

And also not see button "download map" in collector apps mobile
I using system ArcGIS Enterprise 10.7.1.
I has try on ArcGIS Online is successful.