RMAN-06091: no channel allocated for maintenanc...

RMAN> delete obsolete;

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of delete command at 09/27/2012 18:22:20
RMAN-06091: no channel allocated for maintenance (of an appropriate type)

We need to find out whether tape device or disk type using list backup command.
and then if you want to delete disk type backup files, you can use following command.

RMAN> allocate channel for maintenance type disk;
RMAN> delete obsolete device type disk;

参考：

http://mohamedazar.com/2011/05/16/rman-06091-no-channel-allocated-for-maintenance-of-an-appropriate-type/