One scenario in which this error occurs is when a cluster at a higher version is partnered to a cluster at a lower version that does not support multiple cluster mirroring, and you attempt to create another partnership to a cluster at the higher version to implement multiple cluster mirroring. Adding a partnership to a third cluster is not supported while at least one cluster in the current partnership is at the lower version.
Either update 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.