OSError: MoviePy error: the file guitar.mp4 could not be found

@convert.route('', methods=["POST"])
def convert_mp4():
    if request.files.get("mp3"):

        # clean the filename
        safe_filename = secure_filename(request.files["mp3"].filename)

        # save file to some directory on disk
        request.files["mp3"].save(os.path.join("/path/to/some/dir", safe_filename))
        video_clip = VideoFileClip(os.path.join("/path/to/some/dir", safe_filename))
        audio_clip = video_clip.audio  # convert to audio

        # you may need to change the name or save to a different directory
        audio_clip.write_audiofile(os.path.join("/path/to/some/dir", f"{safe_filename}.mp3"))

        # close resources, maybe use a context manager for better readability
        video_clip.close()
        audio_clip.close()

        # responds with data from a specific file
        send_from_directory("/path/to/some/dir", f"{safe_filename}.mp3")
    else:
      return jsonify(error="File 'mp3' does not exist!")