If SO is incomplete, then automatically set the delivery block on the SO header.
as suggested by earlie conutants, u need to make use of user exit
MV45AFZZ, at save document prepare,
Here you can ask the abap consultant to write the code where it checks the incomplete status of the document, if incomplete it updates delivey block to the document or line item as required.
fields concerned
VBUK - UVVLK ( Header incompletion status concerning delivery)
OR
VBUK - UVVLS ( Header incompletion status concerning delivery)
And if the status is not equal to C or " "
update field
VBAK - LIFSK
