[RMAN8i] Backups retention

[Latyr]

Hi everybody and Happy New Year

I have the following problem : I want to delete some of my old backupsests. The script below is the spool of an SQL script that is run against RMAN.
May some one help me to understand.

Thanks in advance.

Gestionnaire de récupération (Recovery Manager) : version 8.1.7.4.1 - Production

RMAN-06005: connecté Ã* la base de données cible : LVS (DBID=3646182293)
RMAN-06008: connecté Ã* la base de données du catalogue de récupération

RMAN> # Resync the catalogue
2> resync catalog;
3> allocate channel for delete type disk ;
4> run {
5> change backupset 459 delete ;
6> change backupset 503 delete ;
7> change backupset 908 delete ;
8> change backupset 922 delete ;
9> }
10> release channel;
11> # Resync the catalog
12> resync catalog;
13>

RMAN-03022: compilation de la commande : resync
RMAN-03023: exécution de la commande : resync
RMAN-08002: lancement de la resynchronisation complète du catalogue de récupération
RMAN-08004: resynchronisation complète terminée

RMAN-03022: compilation de la commande : allocate
RMAN-03023: exécution de la commande : allocate
RMAN-08030: canal affecté : delete
RMAN-08500: canal delete : sid=34 devtype=DISK

RMAN-03022: compilation de la commande : change
RMAN-03023: exécution de la commande : DELETE
RMAN-03026: reprise d'erreur libérant des ressources canal
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03006: non-retryable error occurred during execution of command: change
RMAN-07004: unhandled exception during command execution on channel delete
RMAN-20503: DELETE cannot delete expired objects - run CROSSCHECK or DELETE EXPIRED

Gestionnaire de récupération (Recovery Manager) terminé.

[akhadar]

The following error says all
RMAN-20503: DELETE cannot delete expired objects - run CROSSCHECK or DELETE EXPIRED