Ladder is missing the side reference edge object which is a required input for a ladder with the selected top support. Edit the ladder and select a reference edge. (Structure)
Meaning: A ladder requires a side reference edge object if it is placed using a surface or plane as top support. This ladder's side reference edge object is missing, more than likely because the side reference edge object was deleted.
Recovery:
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Open the Structure task.
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Click Select .
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Set the Locate Filter to Ladders.
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Select the ladder with the error.
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Click Select Reference Edge on the ribbon.
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Select a new reference edge object in the model.
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Click Finish.