Thanks for replying me. I got most of our doctors on the list. There is one doctor. I added her in to demobooking.xml. It shows that her name is "walk-in". I give her an alias and add it into the question. I do not know why.
Also, it never shows up available appointments. I added a new user as "selfbook" into our system. I log in with selfbook and it can search for other slots. But it never shows empty slots on the "online booking" page.