Hec GeoHMS Longest Flowpath

[AmandaPenning]

Hello,

I'm trying to run the Longest Flowpath tool in the Hec GeoHMS toolbar.  I've done all the set up according to the instructions I was given and had no problems with the previous steps.  However, on this step I keep getting the error shown in the image.  Is this a known problem or am I doing something wrong?