delete backup pieces older than 15 days

Version oracle 12.2 Enterprises

I have backups on the disk older than 15 days but command came with no match

RMAN> crosscheck backup;

RMAN> DELETE BACKUP COMPLETED BEFORE 'SYSDATE-20';

using channel ORA_DISK_1

using channel ORA_DISK_2

using channel ORA_DISK_3

using channel ORA_DISK_4

specification does not match any backup in the repository

RMAN> exit

any one know how to delete backup pieces (only not archive)

many thanks