Why my chin have a bald spot and how can I get it to grow? (photo)

There is no way to explain why you have a small bump on your chin. If you are concerned you should see a doctor for an exam.  The opinions or information on the Internet would be at best guesses and it would not lead to a solution or a treatment.

There are a a number of causes of hair loss on the chin in this manner. However, if there is a bump that is persisting a dental examination of the lower jaw and gums is also required (to evaluate for connections and dental sinuses) in addition to seeing a dermatologist specializing in hair loss.  You'll need a panel of blood tests as well.It's not possible to conclude what is happening with this photo. But a dermatologist can help you sort it oit.

This is the type of presentation that requires you to visit a doctor. From the photos, it is not possible or reasonable for me to render an opinion.