When using trace upstream/downstream query in Navigator, following error is seen.
This error is seen when upstream/downstream network is disconnected due to disconnected nodes in network or due to inactive elements in between the network like seen below (blue color elements). If you validate the model to check for errors it will direct you to those elements in network due to which disconnection of network is seen.
After connecting these disconnected nodes to the network or making these intermediate elements active, the error should be gone.
Using Network Navigator