Not able to delete delivery document

Pls help me out for the below mentioned scenario.

1. Delivery document created with handling units. PGI done for the Dly and then reversed.

2. I need to delete the dly. document. While deleting the Dly. document getting an error, handling unit also to be deleted.

3. But I am not able to delete the HU also. System error "HU 1000670037 cannot be changed & process cancelled."

Pls reply.

Regards,