it took me some time to figure out a very anoying error when generating watersheds in ArcGIS 10.5.1.
If the field type of the pour point data is double, and if there is no other integer field in the attribute table, the watershed command will not run and the following error message is displayed:
ERROR 999998: Unexpected Error.
Failed to execute (Watershed).
The same error occurs if you try to convert the point feature class to a raster.