Available with Network Analyst license.
The network dataset has an unknown coordinate system. The results of any network analysis performed on the network dataset will also have unknown coordinate systems.
Any layer with an unknown coordinate system may not be drawn if other layers in the service have a coordinate system. The analysis results can't be converted to another spatial reference.
Define a coordinate system for the source feature classes of the network dataset and rebuild the network dataset.
Do nothing. You can choose to accept the drawbacks and ignore the warning.
When you specify the coordinate system, the network can be accurately reprojected to match the projection of other map layers that might be displayed in a common view. The analysis results can also be converted to another spatial reference.