During playback, select the "Closed Caption or Subtitles" icon from the playback menu. You may need to select "English [CC]" from the list.
While the show is in English, specialized subtitles (hearing-impaired) or local language subtitles are usually provided by the streaming provider (e.g., Netflix, Prime) rather than needing external download links. Watch The 100: Season 6 | Prime Video - Amazon.com