Are you getting this message when assigning a tram / train to a line?
In case it is a tram, check is the tram depot connected to tramway.
In case it is a train, see if the train can go to any station in the assigned line without flipping the train.
A new train will not leave depot unless it has a direct route to its assigned line.
OK: Depot > assigned station
NG: Depot > any not assigned station and make a U-turn > assigned station