Layer's data frame is required to have known coordinate system for packaging.
Solution
Select known coordinate system for the layer's data frame before creating a layer package.
More information
When packaging content, it is important to provide a known coordinate system for the layer's data frame. To add the coordinate system, right-click on the layer in the ArcMap table of contents and select Properties. Click the Coordinate System tab on the Data Frame Properties dialog box, then specify the coordinate system to use.