Once you select a teacher and then fill out the form, before checking out, scheduling a time with a teacher does not allow you to pick a date or time on the next pop up screen
Fixed - Scheduling Lessons
Do you see the Schedule Lesson button? Or are you clicking it, going to the teacher's schedule, but finding that the teacher has no available time slots? "available for lesson" is shaded blue, as indicated in the key above.
I'm reading the Malazan Book of the Fallen.