Error on Stream Reaches and Catchments

[Jesus Sanchez]

Hello,

I am running the Stream and Catchment tool but I keep getting an error (error message attached as a picture). The watershed contains a medium size municipality and more than half of the watershed is located downstream of a bluff next to the Mississippi River. 

I am working on 3 watersheds (HUC12s) but only this one is giving me that error. I am guessing it's because it's mostly flat and it is having a hard time figuring out where the water flows?