No,we cannot assign the incompletion procedure to the billing documents.
We can assign it to the sales document types,item categories,schedule line categories,partner functions,sales activities,delivery types and delivery item types.
We cannot assign why because the billing document is always created with reference to another document i.e. either sales order or delivery document.It cannot be created independently.So if the previous document is incomplete,you cannot create the billing document.
Please follow the path for assigning the incompletion procedure.
>Sales and Distribution>Basic Functions >Log of Incomplete Items>Assign Incompleteness Procedures.
It is not possible to assign incompleteness procedure to billing document.
Eventhough if you want to make some fields mandatory, such that without those billing should not save, use USEREXITs at billing document level and check for those fields.
Check with ABAPer you can have screen userexit and save userexits for this.
try and revert