I was looking at this to see if it could give me any insight into my own error. (I'm having a strange problem with raster output in flood plain delineation.) While looking it up, I saw this thread: https://community.esri.com/thread/102706
Maybe that could help you? At the very least, the top comment lists an email address that you could send an email too. If I get the chance, I'll try to run through an attempt with the tool and see if I can at least duplicate the problem. I'm wondering if there's a compatibility issue with ArcMap 10.6.1, but right now, that's only a suspicion.