Some viewing and data collection (disconnected editing) instances require web maps and map layers to be available for offline use.
To provide for this capability, some preparation is required. Several options exist, but a recommended option is to prepare the web map and provide areas for download before field workers use the web map (and map layers).
Make sure to review the sections on data and map requirements and setting the offline settings and manage map areas for use in offline mode.
Access the Offline Mode in the Web Map Settings
Click on the Item Details of the Web Map of interest (often found in My Content) or a Group where the member has the permission to change the settings of the web map.
Change the Settings for Offline Mode
This section shows where to toggle the button to set the offline mode as well as create and manage areas to download for one or more areas.
This video shows shows how to set some of these options in ArcGIS Online.
This video shows how the the areas can be downloaded for offline use.