Interesting comment but there really aren't any valid tips to fix ESRI:s 999999 Error. This error, by definition, is "unexpected" and without a known cause.
With respect to the actual question posed by ArminAzad, the error arises when running a Fill on a DEM.
When faced with en error in this situation, the best course of action is typically to reduce the size of the input. I.e. use ClipRaster to reduce the DEM by 50% and then run the Fill. Does it still crash? If not, then the problem has been identified. However, one thing I would never recommend is to try and run RepairGeometry on a raster elevation model 😬