The layer's map must have a known coordinate system for packaging.
Select a known coordinate system for the layer's map before creating a layer package.
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.
For more information, see Analyze your GIS resource.