Playback Error occurs when trying to play video after editing its file name

Correct.

If you follow the documentation, it will work 99% of the time. There is always the occasional movie or show that gives Plex trouble, but they are the exception, not the rule.

Remember, if you try something and it does not work, nothing bad happens. Plex does not delete the files. Flames do not shoot out of your server. Worst case is you try something else and maybe have to Plex Dance the files.

Plex DocumentationYour MediaAdding Local Subtitles to Your Media

Plex docs have examples for both movies and TV shows.

  • External subtitles must be in the same folder as the media file.
  • Add the appropriate language tag to the file. Otherwise, Plex will display the languauge as “Unknown.”
  • The subtitle file must be named exactly the same as the media file, except for language and other tags.

Many people keep resolution, codec, etc info in the media file name. If you do so, it must also be in the subtitle file name. Place such info inside square brackets at the end of the file name.

Movie example. The same applies to TV shows.

/movies
  /Avatar (2009)
    /Avatar (2009) [1080p.HEVC.DD51].mkv
    /Avatar (2009) [1080p.HEVC.DD51].eng.srt
    /Avatar (2009) [1080p.HEVC.DD51].eng.forced.srt

Naming and Organizing Your TV Show Files

Episodes belong directly in their season folder.

Look at the documentation. The example at the bottom of the page shows how to organize the files.