Either the source VDisk or the target VDisk, or both, are already members of other FlashCopy mappings. The FlashCopy mapping was not created because the new FlashCopy mapping that you attempted to create would have linked two existing mapping trees into a single tree that exceeds the maximum number of mappings that are supported for a single tree.
You have two options. The first option is to resubmit the command and specify a different source or target VDisk. The second option is to delete a sufficient number of the existing FlashCopy mappings in which either the source or the target VDisk is a member so that the combined mapping tree does not exceed the maximum number of mappings that are supported for a single tree, and resubmit the command.