All FlashCopy mappings that are in a tree of connected mappings must have the same grain size. The new FlashCopy mapping that you attempted to create would have linked two existing trees that have different grain sizes.
You have three options. The first option is to resubmit the command and specify a different source or target VDisk. The second option is to delete all of the existing mappings that contain the source VDisk and resubmit the command. The third option is to delete all of the existing mappings that contain the target VDisk and resubmit the command.