Meaning: The reference plane or axis is missing for positioning the nozzle. Someone might have deleted the shape you placed the nozzle on.
Recovery:
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Set the Locate Filter to the nozzle type.
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Select the nozzle in error.
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Click Edit > Properties.
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Click the Location tab.
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Change the nozzle Placement Type to Position by Point.
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Define the location of the nozzle in the model.