I think i've encountered such an error before.
I can't remember which solution helped me but try these 3 in separate projects so you can avoid more errors:
- Change the projection of the original DEM that you started the project with. If it's Geographic then project it to a projected coordinate system and vice versa.
- Try putting the output file in the default geodatabase. If that didn't work try putting it in the default geodatabase under a file named " Layers " .
- Try putting Inlets and Outlets instead of just an Outlet.
Hope this helps.