As Marija says... not all teachers offer trial lessons. You can click on "Professional Lessons", a window will open where you can select "Teaches" (language that the teacher teaches), "Also Speaks" (what languages the teacher speaks), "From" (place where the teacher is), "Price" and in the lower part you can select "Native Speaker", "Trial Lesson" (you should click on this one, so that the system shows you who offers trial lessons) and finally "video class available". I hope you can find a suitable teacher. 気を付けてください

Not all teachers offer trial sessions, so that might be the reason you couldn't schedule one. Take care :)