Description
There is no vertical unit (z-unit) in the input's spatial reference. To perform geodesic analysis, a z-unit is required.
Solution
You can define a vertical coordinate system with the expected vertical units for your input using the Define Projection tool.
If you use an input that does not have a vertical unit defined, meter will be used by default as the z-unit. You can also choose another unit from the parameter drop-down list.