I believe you can no longer do this on-line. I had the same issue. There is an 800 number on that page to call. No long hold time a month ago. I was avoiding calling due to reports of long holding time, but if I recall correctly, it was for magical express only.
Are you trying to schedule for an arrival date this calendar year (2021) or next year (2022). DME is ending and is only available for arrivals through 12/31/2021.
For me it is for November. The first few numbers were right but the last half was wrong. So I just went with what was closest. I'll probably call to confirm next week just to make sure they got it.