All FlashCopy mappings that are in a tree of connected mappings must have the same grain size. The FlashCopy mapping was not created because the source VDisk that you specified is either the source or the target VDisk of another FlashCopy mapping, and the grain size of the other mapping is different from the grain size that you specified for the mapping that you attempted to create.
You have two options. The first option is to delete all of the FlashCopy mappings that contain the source VDisk that you specified where the grain size of the FlashCopy mapping is different from the grain size that you specified, and resubmit the command. The second option is to resubmit the command and do not specify the grain size attribute.