Here are all the messages that I got from my execution of Flow Direction, trying to use the new Multi Flow Direction algorithm in ArcGIS 10.6.
Executing: FlowDirection C:\Users\A02150284\Documents\GISData\NewBRATFile\01_Inputs\04_Topo\DEM_1\Filled_DEM.tif C:\Users\A02150284\Documents\GISData\NewBRATFile\01_Inputs\04_Topo\DEM_1\Flow\FlowDir.tif NORMAL # MFD
Start Time: Wed Feb 28 11:22:03 2018
ERROR 999999: Error executing function.
Unsupported pixel type
Failed to execute (FlowDirection).
Failed at Wed Feb 28 11:22:03 2018 (Elapsed Time: 0.05 seconds)
I've ran the tool on multiple DEMs, and none have worked. The only tool I've run on the DEM before this is running Fill on it. I haven't seen anyone else have a problem with it, though that makes sense given how new it is. If anyone has advice for how to fix this, I'd appreciate it.