Asset is missing the required 'track' tag.
This error is specific to legacy (circa 2001-2003) assets which require a 'track' tag to indicate the KUID of their attached track splines. This usage ceased with the introduction of the attached-track container in KIND SceneryWithTrack. The typical cause of this error is in fact a missing attached-track container, resulting in the game attempting to parse the asset as a faulty / legacy asset instead.
For assets with a trainz-build version below 2.5, the correct fix is to add an appropriate "track" tag.
For newer assets, the correct fix is to add an attached-track container.