Duplicate Target database for standby from active database failing with ORA-27101

Problem:

Duplicate Target database for standby from active database failing with ORA-27101

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of allocate command at 03/26/2023 14:47:54
RMAN-06403: could not obtain a fully authorized session
RMAN-04006: error from auxiliary database: ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory

Solution:

Inside the listener.ora file in the Standby DB side under SID_LIST_LISTENER the GLOBAL_DBNAME & SID_NAME parameters were set to the db_unique_name value of the standby DB which is DWHSBY while it should be set to the instance_name which is DWH

Wrong:

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC = (GLOBAL_DBNAME = DWHSBY ) (ORACLE_HOME = /u01/oracle/11.2.0.4) (SID_NAME = DWHSBY ))
  )

Corrected to:

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC = (GLOBAL_DBNAME = DWH ) (ORACLE_HOME = /u01/oracle/11.2.0.4) (SID_NAME = DWH ))
  )

Also make sure the tns entry inside the tnsnames.ora file at the Primary DB side is configured to connect to the same SID_NAME configured in the Standby's listener.ora

DWHDB01S =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = STANDBYSERVER)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = DWH)
    )
  )