Film marks are not being detected by the film position sensor.
If calibration fails with a new roll of film, REPLACE PRINTER. The sensor is bad.
If calibration passes, try to clean the sensor with canned air. It is at the top-right of the film area. A calibrated, clean printer with a straight roll of film should not get this error.
The film may have walked (or telescoped) to one end of the film cassette. If the site has an upgraded film cassette, follow the adjustment procedure.
Older film may also have this problem more frequently. If the site has any red dot film they should use that.