Hello,
I hope you all are doing well.
When you go to register for a series and there are a lot of sessions in it scheduled to occur at many distinct times and days of the week, sometimes the little elements in the "SCHEDULE" box that provide the days of the week and the times during which sessions are hosted can push the green "Register" button off the screen, making it so you cannot click or see it. For me, it happens for this series: https://schoolhouse.world/series/82150. It might not be the same for everyone though because of differences in device screen size.
Let me know if you all need screenshots or more information. (=
P.S. I tried to post report this bug a bit ago, but I do not think it went through because it was not under my posts. I am sorry if just missed something and this turns out to be a duplicate.