Symptom
You are executing client copy but getting below error,
Solution
Execute transaction code SCC4. Select the respective client. Ensure below changes are set.
- Client role is not assigned to Production
- Automatic recording of changes
- Changes to Repository and cross-client customizing allowed
- Protection level 0: no restriction
- Deselect Locked due client copy under Restrictions
Also, please make sure if client role is SAP reference by any chance. In my case, in 000 it was complaining for Production client role even when my client role was SAP refernce, when i changed it to test, it worked.