This error message indicates that the meter missed a depth reading. This can happen when:
a) The speed of insertion exceeds the meter's ability to measure depth
b) An obstruction comes into the depth sensor's field-of-view.
You have the option of saving this profile anyway by pressing the Review button. To avoid this message, apply a uniform amount of measurement at a slow speed while measuring, and be sure the depth sensor's field-of-view is clear of obstructions. A description of the meter's field-of-view is is given in the product manual.