Description
This error occurs when evaluating spatial relationships between features stored in different layers. If the layers do not have matching vertical units (feet, meters, and so forth), this error will occur.
Solution
If layers A and B have a common vertical datum (base) for their z-unit, simply copy B to B', then run the Adjust 3D Z tool on B' to convert the z-coordinate.
If layers A and B do not have a common vertical datum, use the Project tool, with B as input; import A's coordinate system into the tool's Output Coordinate System parameter; then write the output to B'.