While taking backup by connecting to catalog database, Got RMAN-20001 : target database not found  error.

Below is the exact error;

released channel: t1
released channel: t2
released channel: t3
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of crosscheck command at 11/24/2015 15:40:53
RMAN-06004: ORACLE error from recovery catalog database: RMAN-20001: target database not found in recovery catalog

RMAN> register database
2> ;

This is because, the target database is not registered in the catalog. So connect to catalog and register the same.

oracle@B2C-RBM-DEV-ST:/home/oracle/RMAN/B2CRBMT2#rman

Recovery Manager: Release 11.2.0.4.0 - Production on Tue Nov 24 16:01:27 2015

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

RMAN> connect target sys/oracle@target_db

connected to target database: TARGET (DBID=30781588877)

RMAN> connect catalog rman_usr/rman@cat_db

connected to recovery catalog database

-- Now fire register command

RMAN> register database
2> ;

database registered in recovery catalog
starting full resync of recovery catalog

full resync complete