Q: Why can I not add "Shri", "Chhatrapati", or "Maharaj" to Shivaji's name?
A: As per WP:Honorifics and WP:Commonname, in Wikipedia, we refer to people by their commonly used name in the cited academic sources. We do not include titles or Honorifics unless they are commonly used in the cited sources (Shivaji does not fall in this category).
Please note Charles III, where he is referred to as "Charles" throughout, not "King Charles" nor "His Royal Highness" or whatnot. We clearly note Shivaji's titles in the article, we just don't repeat them each time his name is written.
Please refer to this talk discussion for more details.