Description
The z-unit may be incorrect or there may be no z-unit.
Solution
If a z-unit is available in the vertical coordinate system of the input raster, it will be applied automatically. If no z-unit is available, set the Z unit parameter value to the corresponding z-unit that matches the input data. Alternatively, define a z-unit for the input raster if it is missing in the input surface raster.
Use the Define Projection tool to specify a z-unit.