Replication Manager was managing a pair that someone tried to start outside of Replication Manager. The copy set pair must be started inside Replication Manager for the operation to complete successfully.
Start the pair in session VALUE_1 for copy set VALUE_2 with source VALUE_3 and target VALUE_4 inside Replication Manager. It is advisable to determine who or what started the pair outside of Replication Manager to prevent the same conflict from recurring.