RMAN-06091: no channel allocated for maintenance (of an appropriate type)

RMAN-06091: no channel allocated for maintenance (of an appropriate type)
The error message appears when we try to delete some backups. Set of commands could be like below.

RMAN> show retention policy;
RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 7 DAYS;
RMAN> delete obsolete;

The reason is as it shows - the appropriate channel.

First we need to confirm if the delete is for Tape or for Disk. Then we need to allocate a maintenance channel for the related device.
Suppose we are deleting from disk, the command to allocate a channel is as below.

RMAN> allocate channel for maintenance device type disk;

Second while deleting we need to specify from where we are deleting the obsolete backup sets, like below.

RMAN> delete obsolete device type disk;