RMAN ORA-12520 Connect Error Tips

rman TARGET sys/syspwd@db  

RMAN-00554: initialization of internal recovery manager package failed
RMAN-04005: error from target database:
ORA-12520: TNS:listener could not find available handler for requested type of server

At the same time, I am able to login using sqlplus (sqlplus sys/syspwd@db as sysdba) without any issue.

Answer:  In your case, you can connect to the db service, but not to the cannot connect to your RMAN catalog instance (named TARGET).  

First, make sure that you have created your RMAN catalog database.  

To see what to do with the ORA-12520 error, you need to start by learning how to use the oerr utility to look-up error messages and actions.  

See here, the cause and action for the ORA-12530 error:

oerr ora 12540
  
ORA-12520:  TNS:listener could not find available handler for requested type of server  

Cause: None of the known and available service handlers for requested type of server (dedicated or shared) are appropriate for the client connection.  

Action: Run "lsnrctl services" to ensure that the instance(s) have registered with the listener and that the appropriate handlers are accepting connections

As we see, you start by running the "lsnrctl services" command and examine the output to ensure that your TARGET service has a handler:

lsnrctl services     - lists all servers and dispatchers
 
[oracle]seagull: lsnrctl services
LSNRCTL for HPUX:Version 2.0.15.0.0-Production on 16-SEP-94 15:36:47  

Copyright (a)  Oracle Corporation 1993. All rights reserved.  

Connecting to (ADDRESS=(PROTOCOL=TCP)(HOST=seagull)(PORT=1521))
Services Summary...
   tdb000                has 4 service handlers
     DISPATCHER established:1 refused:0 current:2 max:55 state:ready
       D001 (machine: seagull, pid: 4146)
       (ADDRESS=(PROTOCOL=tcp)(DEV=5)(HOST=141.123.224.38)(PORT=1323))
     DISPATCHER established:1 refused:0 current:2 max:55 state:ready
       D000 (machine: seagull, pid: 4145)
       (ADDRESS=(PROTOCOL=tcp)(DEV=5)(HOST=141.123.224.38)(PORT=1321))
     DISPATCHER established:0 refused:0 current:1 max:55 state:ready
       D002 (machine: seagull, pid: 4147)
       (ADDRESS=(PROTOCOL=tcp)(DEV=5)(HOST=141.123.224.38)(PORT=1325))
     DEDICATED SERVER established:0 refused:0  

The command completed successfully

See these related helpful notes on fixing the ORA-12520 error:

•  See here how to diagnose SQL*Net connectivity issues.

• See MOSC Note 240710.1 for more details to RMAN and the ORA-12520 error.