STATUS CODE: 6 " the backup failed to back up the requested files"

Hi All;
I need you help to resolve my problem,, it is STATUS CODE: 6 " the backup failed to back up the requested files"  when Run Oracle Database backup using RMAN
and the output LOG for failed joj as follow :-

released channel: ch01

RMAN-00571: ===========================================================

RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============

RMAN-00571: ===========================================================

RMAN-03002: failure of send command at 09/01/2009 15:01:04

ORA-19559: error sending device command: NB_ORA_SCHED=

ORA-19557: device error, device type: SBT_TAPE, device name:

ORA-27194: skgfdvcmd: sbtcommand returned error

ORA-19511: Error received from media manager layer, error text:

   NBBSA_SCHEDULE not found in environment block

kind regards.