I can't solve your larger problem but I can (I think) fix this.
the problem is that you have delitems in 2 places, change to this:

Update Delitems
Set Inv_Nmbr=(Select InvHead.Inv_Nmbr From InvHead
Inner Join DelItems DI on DI.Acno=InvHead.Acno
Where DI.Delivered='N'
And InvHead.Delivered='N');

>Dynamic SQL Error
>SQL error code = -204
>Ambiguous field name between table/view DELITEMS and table/view DELITEMS
>ACNO
>
> Update Delitems
> Set DelItems.Inv_Nmbr=(Select InvHead.Inv_Nmbr From InvHead
> Inner Join DelItems on DelItems.Acno=InvHead.Acno
> Where DelItems.Delivered='N'
> And InvHead.Delivered='N');

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