If you are streaming the movie and the subtitles are visibly broken (e.g., gibberish text) or the option is grayed out, it is likely a bug with that specific streaming service app. Restarting the app or checking for an update usually resolves this.
If you are missing subtitles—whether you need them to understand the dialogue or simply want them for the foreign spoken parts—here is how you can fix this depending on how you are watching the movie: megan is missing subtitles