The download may be corrupted. Re-download the video.
Tools like CloudConvert or Ezgif can convert .mp4 to other formats (e.g., GIF, MOV). Twitter (10) mp4
Sometimes, right-clicking and "saving" the page saves the HTML instead of the video. Use a dedicated downloader. "Codec not supported": Update your media player or use VLC. The download may be corrupted
Use media players like VLC Media Player (plays anything), QuickTime (Mac), or Windows Media Player. a specific app
Sites like twittervideodownloader.com or twdown.net (use caution with ads).
Searching the Chrome Web Store for "Twitter video downloader" offers several options.
did you get this file from (e.g., a specific app, browser, or website)? What is the issue (e.g., won't open, wrong format)? Knowing this will help me give you a better solution!