You are sharing a layer symbolized with unique values symbology, where one or more symbol classes are not visible. All symbol classes must be visible to share the layer.
You can control the visibility of each unique value symbol class independently using check boxes in the layer's Symbology pane.
Solution
Make the symbol classes in the layer visible. In the Symbology pane, on the Primary symbology tab 
, on the Classes sub tab, check the boxes next to each symbol class to make them visible. You can turn on all symbol classes simultaneously by checking the box in the heading of the table. A square in the heading check box indicates that some symbol classes are on and some are off. Click the box until it shows a check mark, indicating that all classes are visible.
Additional information
For more information, see Control the visibility of symbol classes and Analyze your GIS resource.