Ah, it’s probably Plex not supporting the video’s codec. When you use MakeMKV to generate a video file, it’s basically just grabbing the source file directly from the disc / iso. It’s fine in modern players but for maximum compatibility you’ll have to remux it.
EDIT: Forgot DVDs use a different folder structure than BDs so that guide won’t work, you can use this one from thewiki.moe instead (although it’s very technical, and the wiki is moreso focused on anime.) You can maybe try using ffmpeg to convert to an mp4, that might just make it work on Plex.