Please , who can tell me why this hard disk serial number display program is not working?

A hard disk is not a SCSI-2 media changer device. The IOCTL_CHANGER_GET_PRODUCT_DATA call only applies to devices that support the SCSI-2 media changer interface (such as tape libraries). Depending on exactly what you're trying to do, IOCTL_STORAGE_GET_MEDIA_SERIAL_NUMBER might be what you want.