Meaning: The evaluation of the weight and the center-of-gravity failed.
Recovery:
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Open the Structure task.
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Click Select .
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Set the Locate Filter to Slabs.
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Select the slab in error.
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Change the Face Position on the ribbon to a different option and then back to the original option. This action causes Smart 3D to recompute the slab and calculate the weight and center-of-gravity.