RMAN Backup in linux

Have you linked the NBU plugin with Oracle?

See steps in NBU for Oracle Admin Guide for UNIX, Windows, and Linux  http://www.symantec.com/docs/DOC3672

Important to ensure successful forward and reverse hostname lookup between master and client - use hosts file entries if Linux server is not on DNS.
Check that OS firewall (iptables) on client is disabled.
Confirm lookup and connectivity using this command on the client:
/usr/openv/netbackup/bin/bpclntcmd -pn

To troubleshoot, ensure that the following log folders exist:

On master: bprd (Restart NBU Request Service to enable log)

On client: bpcd and dbclient
chmod 777 dbclient
(this is necessary to allow oracle user to write to this folder)

After next backup attempt, check log folders on client as well as rman .out file (see script for filename and location).
Check bprd log on master to see if connection/backup request was received from client.

Lastly - if you are not the Oracle dba, work with your dba to ensure successful backups and restores.