The vertical factor parameter file that you entered either is empty data or is in a format that is not accepted by PathDistance. The vertical-factor graph defines the relationship between the vertical cost factor (VF) and the vertical relative moving angle (VRMA). The vertical parameter file is an ASCII file used to produce the graph. The file should have two columns. The first column identifies the VRMA in degrees, and the second, the VF.
Check that the input vertical factor parameter file has valid content and is in the correct format required for PathDistance.