Hi. I'm going to create a course that is based on a fiction book. Would that be possible? I mean, won't it break any rules here? I haven't seen any similar courses here
You can create a course on pretty much anything, except for the stuff you'd expect to be prohibited:
https://support.udemy.com/hc/en-us/articles/229233027-Restricted-Topics
My concern is copyright law. I don't really know what your course will be about, but if it's based on someone else's fiction book, remember you generally can't use someone else's content without their permission.
Are you the author?
Several of my courses are based on my books, but I own the copyright to all the material. If you are not the copyright holder than you do not have a right to use the author's intellectual property.
I have no idea how you would create a course based on fiction, however.