Meaning: The Sketch 3D path, the trench run slope, or the invert elevation values are causing the trench feature elevation to be a negative value.
Recovery:
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Select the trench run in error.
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Modify the Sketch 3D path in the Graphic View so that each knuckle edge is a positive elevation value.
-OR-
Modify the elevation and slope values in the Properties View so that the start elevation is a positive value and the elevation remains positive along the run.