The code versions of the clusters in the existing partnership do not support a partnership with a cluster that has the code version of the remote cluster with which you are attempting to create a partnership. If a cluster at version 5.1.0 or later is already in a partnership with another cluster at version 5.1.0 or later, you can only add a partnership to a cluster at version 5.1.0 or later, and cannot add a partnership to a cluster at version 4.3.1 or earlier. If a cluster at version 5.1.0 or later is already in a partnership with another cluster at version 4.3.1 or earlier, you cannot add another partnership while the partnership with the cluster at version 4.3.1 exists. If a cluster is not in a partnership, you can create a partnership between it and a cluster at any version. One scenario in which this error occurs is when you attempt to add a partnership with a remote cluster at version 4.3.1 or earlier to a cluster at version 5.1.0 or later that is already in partnership with another cluster at version 5.1.0 or later.
Either update the downlevel cluster code version to a version that supports this task or remove all existing partnerships from the cluster to which you want to partner the cluster that has the downlevel version, and resubmit the task.