Failed to backup database to tape using RMAN and LSM! but can backup
database to disk using RMAN.

Here is the cmd and error log:
scsidev@0.0.0:LG CD-ROM CRD-8483B1.00|CD-ROM
scsidev@1.0.0: |Disk
scsidev@2.0.0:SEAGATE ST318404LC 0002|Disk, \\.\PHYSICALDRIVE4
scsidev@2.1.0:SEAGATE ST318404LC 0002|Disk, \\.\PHYSICALDRIVE5
scsidev@2.6.0:ESG-SHV SCA HSBP M8 0.05|Processor
scsidev@3.3.0:HP C1537A L005|Tape, \\.\Tape0
2.C:> rman rcvcat rman/rman@rmancat target internal/oracle@zbxt cmdfile
Recovery Manager: Release - Production
RMAN-06005: connected to target database: ZBXT (DBID=1586247895)
RMAN-06008: connected to recovery catalog database
RMAN> run {
2> allocate channel tt type 'SBT_TAPE';
3> backup tag 'zbxt_test'
4> format 'zbxt_t%t_s%s_p%p'
5> (tablespace users);
6> release channel tt;
9> }
RMAN-03022: compiling command: allocate
RMAN-03023: executing command: allocate
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03007: retryable error occurred during execution of command: allocate
RMAN-07004: unhandled exception during command execution on channel tt
RMAN-10035: exception raised in RPC: ORA-19554: error allocating device, device
type: SBT_TAPE, device name:
ORA-19557: device error, device type: SBT_TAPE, device name:
ORA-27000: skgfqsbi: failed to initialize storage subsystem (SBT) layer
ORA-19511: SBT error = 4110, errno = 0, BACKUP_DIR environment variable is not
RMAN-10031: ORA-19624 occurred during call to
Recovery Manager complete.

By the way,can someone tell whether or not I have to register LSM ?I 'm using the LSM inside the Oracle CD.