Normally, for any reason, if billing documents cannot be generated, it could be due to incompletion log exists in sale order and not in delivery. So in all probabilities, wherever system has stopped generation of billing document, if you check the corresponding sale order, it would have been saved with incompletion log. So before executing VF04, user has to educate to check this. On the other hand, if you want to automate this process, you can try with USEREXIT_FILL_VBRK_VBRP in program RV60AFZC
G. Lakshmipathi