The authors who offer their material here decide about what is included in their teaching material themselves. So they decide individually for each material whether to provide a solution sheet or not. Some authors decide not to publish solutions because they might be viewed and used by students.
However, if a material does include solutions, it will be noted in the description of the material.
If you would like the solution to be added, you can contact the author directly under "Reviews and Comments for this material" and ask for it. If they decide to add it, you will be able to download the addition for free.