Problem : You are not able to create billing document in SAP.
Solution : This could happen because of multiple reasons. Some probable causes are listed below.
Reason # 1 : You are trying to create the billing document from the Sales Document ( Sales Document -> Deliver ) whereas the delivery needs to be created with respect to a delivery ( as opposed to order )
Reason # 3 : Item category not Relevant for Billing in SAP. For example, standard item category TAN is relevant for billing, but if you use some other item categories, the item category might not be relevant for billing.
Check the billing relevance of the item category in tcode [ VOV7 ] . If the item category is relevant for order relevant billing do a order relevant billing, or if it is relevant for delivery relevant billing, do delivery relevant billing. Know more about SAP Item Category and SAP Item category Determination