An enclosure cannot be managed by more than one cluster at a time.
If you do not have access to the cluster that is currently managing the enclosure, you can log on to the enclosure and run the satask leavecluster -force command. Using the -force parameter can lead to data loss if the enclosure is in use.
After you remove the enclosure from the management of the previous cluster, you can retry the original chenclosure -managed yes command.