The number of FlashCopy mappings in which a volume can be defined as the source volume is limited. The source volume that you have specified cannot be defined to another FlashCopy mapping because it is already defined as the source volume to the maximum number of FlashCopy mappings.
You have two options. One option is specify a different source volume and resubmit the command. The other option is delete one of the existing FlashCopy mappings that contains the source volume and resubmit the command.