There are two methods to lock and unlock a client which is locked while performing a client copy.
Steps:
Option 2: TCODE: SE37
1) Login to the any client with "DDIC" or "SAP*" and execute SE37, enter "SCCR_UNLOCK_CLIENT" to unlock or "SCCR_LOCK_CLIENT" to lock a client. Click the "Execute" button / F8
2) Enter the client number to be unlock and click "Execute" button / F8
3) Now the option "Locked due to client copy" can be checked list
3) Now the option "Locked due to client copy" can be checked list