I can't find which required field is missing on the online application screen for "Omatome Seikyu Service (combined billing method)" or "Web Billing."
回答
You will see the required fields filled in pink (sakura) for "Omatome Seikyu Service (combined billing method)" and "Web Billing."
Please check if you have input all fields filled in pink.
Also, it is required to put a check in the box on the bottom of the screen for agreement to 'terms and conditions in terms of "Omatome Seikyu Service,"' or 'terms and conditions in terms of "Web Billing."'