The horizontal factor graphs define the relationship between the horizontal cost factor (HF) and the horizontal relative moving angle (HRMA). The horizontal parameter file is an ASCII file used to produce the graph. The file should have two columns on each line. The first column identifies the HRMA in degrees, and the second, the HF.
The horizontal factor parameter file that you entered either has no data or is in a format that is not accepted by PathDistance. The documentation for How PathDistance works gives more information on the formatting of the parameter file.