The unit did not recognize the strip or target that was read.
Was the correct strip definition displayed in the unit's window prior to the reading?
Select the correct strip by pressing key #4 under the word "other" until proper strip is displayed.
Are there any measurement patches within the strip that are damaged, cloudy or have excessive gradients?
Try re-reading strip or print out a new target or strip.
Does the strip pass through the unit without slipping or hesitation? Contaminated drive wheels or obstruction in target's path, try cleaning with compressed air following the
instructions in the Operations Manual.