The layer's map must have a known coordinate system for packaging.
Solution
Select a known coordinate system for the layer's map before creating a layer package.
More information
When packaging content, it is important to provide a known coordinate system for the layer's map. To add the coordinate system, right-click the map in the Contents pane and click Properties. On the Map Properties dialog box, click the Coordinate System tab, and specify the coordinate system to use.
Additional information
For more information, see Analyze your GIS resource.