Return delviery can't be created?

Hi experts

We create return po and mark that vendor as return vendor in vendor master. then we relase this po.

But when we try to use vl10b to create delivery, it always said " Document is incomplete: You cannot save the delivery. When the system checked whether saving delivery was permitted, it determined that the requirements stored in Define Incompletion Procedures for this document do not allow this subsequent function."

Any advice why it can't create delivery? Or any configure restriction?

Thanks

Alice