Can you write a book about something you have never experienced or witnessed?

That’s known as an author’s due diligence, they either draw from their related experiences, or they do the necessary research required to accurately convey the story they’re writing. If you’re incapable of doing that, you can hardly call yourself an author, as it’s a necessary part of the job, just as being able to type is essential for most jobs. If you can't do it, few others than day laborers will hire you.