You cannot initialize or create a system on a canister whose partner is already defined as part of a system. A system ID is defined when a system is initialized or created, and a system ID is already defined for the partner canister.
Ensure that the partner canister of a canister on which you attempt to create a system is not already defined as a member of a system. See Resolving a problem topics in the Troubleshooting section of the Information Center.