56 FLIGHTMODE Follow line: line is too far away

This is a safety massage that appears if AirDog's takeoff position is farther than 500 meters (1,640 feet) away from closest point on the virtual line:

If you are sure that the takeoff position is closer than 500 meters to the virtual line, please sync the virtual line again via the AirDog mobile App. 

Here's a description on how to create the virtual line.