Your map does not contain a basemap. Your map must contain a basemap to share it as a web map.
Add a basemap.
- On the Map tab, in the Layer group, click Basemap.
- Choose a basemap from the gallery.
The basemap's tiling scheme defines the spatial reference of the web map. All cached web layers must use the same tiling scheme as the basemap to ensure they align correctly.
Learn more about how to author a custom basemap and specify a coordinate system.
For more information, see Analyze your GIS resource.