The backup failed to back up the requested files (6)

Hello Team,

We are getting error on RMAN Backup.. please suggest what is the issue with it and what can be done to resolve it..Thanks.

Master, Media : SunOS 5.10 - NBU 7.6.1 

Client : SunOS 5.10 - NBU 7.6.0

Details Status :

01/23/2018 03:34:13 - Error bptm (pid=24606) error requesting media, TpErrno = Robot operation failed

01/23/2018 03:36:03 - Warning bptm (pid=24606) media id XXXXX load operation reported an error

01/23/2018 03:36:03 - current media XXXXX complete, requesting next media Any

01/23/2018 04:03:05 - Info dbclient (pid=31973) done. status: 6

01/23/2018 04:03:05 - Error bptm (pid=24606) media manager terminated by parent process

01/23/2018 04:03:06 - Info dbclient (pid=31973) done. status: 6: the backup failed to back up the requested files

01/23/2018 04:03:06 - end writing

the backup failed to back up the requested files  (6)

RMAN :

channel ch08: starting piece 1 at 23-JAN-18
RMAN-03009: failure of backup command on ch11 channel at 01/23/2018 01:24:40
ORA-27192: skgfcls: sbtclose2 returned error - failed to close file
ORA-19511: Error received from media manager layer, error text:
   Failed to process backup file <bk_ufbspbq5h_s551403_p1_t966125745>
ORA-19502: write error on file "bk_ufbspbq5h_s551403_p1_t966125745", blockno 16247297 (blo
cksize=512)
ORA-27030: skgfwrt: sbtwrite2 returned error
ORA-19511: Error received from media manager layer, error text:
   VxBSASendData: Failed with error:
channel ch11 disabled, job failed on it will be run on another channel