backup problem

I'm tryiong to change my setting-backup but I optaint the following error.

Futhermore, the full backup has the same error.

Someone knows how to repare the problem?

Thanks in advance.

Marco

ecovery Manager: Release 10.2.0.5.0 - Production on Thu Dec 2 14:42:55 2010

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RMAN>
connected to target database (not started)

RMAN>
echo set on

RMAN> CONFIGURE CHANNEL DEVICE TYPE DISK clear;
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of configure command at 12/02/2010 14:42:56
RMAN-06403: could not obtain a fully authorized session
ORA-01034: ORACLE not available

RMAN> CONFIGURE CHANNEL DEVICE TYPE 'SBT_TAPE' clear;
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============