One scenario in which this error occurs is when a cluster at version 5.1.0 or later is partnered to a cluster at version 4.3.1 or earlier and you attempt to create another partnership to a cluster at version 5.1.0 to implement multiple cluster mirroring. Software version 4.3.1 does not support multiple cluster mirroring, so adding a partnership to a third cluster is not supported while at least one cluster in the current partnership is at version 4.3.1 or earlier.
Either upgrade the downlevel cluster software version to a version that supports this task or remove the partnership to the cluster that has the downlevel software version, and resubmit the task.