first check have you completed the PGI for the delivery document or not.
Next, go to that Item Category functionality VOV7 and find if it is order or delivery related billing.
And after that go and check in the particular Sales order document type whether billing document types are maintained in the sales document or not.
If every thing is correct go and find with copy controls
If it order related, go VTFA
If it Delivery related, go VTFL and check in your document type combination is maintained or not, if they are maintained item category is available or not.
Check if Item Category is checked for Billing or not
Normal Sales OrderCycle:-