RMAN-06174: not connected to auxiliary database

Problem:
When duplicating from an active database I was getting this error:

...
sql statement: alter system set  db_name =  ''TESTDB2'' comment= ''Modified by RMAN duplicate'' scope=spfile

Oracle instance shut down

released channel: disk1
released channel: disk2
released channel: disk3
released channel: disk4
released channel: disk5
released channel: disk6
released channel: disk7
released channel: disk8
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 08/01/2019 14:24:24
RMAN-06174: not connected to auxiliary database
RMAN-03015: error occurred in stored script Memory Script
RMAN-04006: error from auxiliary database: ORA-12514: TNS:listener does not currently know of service requested in connect descriptor


Analysis:
The new instance was not properly registered with the listener on the destination server.
Registering the destination instance statically with the listener [on the destination server] has fixed the issue:

Solution:
In the destination/target server Add the lines in green color to the listener.ora file with the right destination/target instance name:
LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 172.5.5.21)(PORT = 1521))
    )
  )

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = AWSUAT4DB2)
      (ORACLE_HOME = /u01/app/oracle/product/11.2.0/dbhome_1)
    )
  )

Then reload the listener: [On the Target/Destination Server]
# lsnrctl reload