Why MKV is sometimes rejected
MKV can contain different video codecs, audio tracks, subtitle tracks, and attachments. That flexibility is useful, but many websites and devices prefer MP4 for predictable playback.
MP4 is the safer upload format
Convert MKV to MP4 when a video needs to work on phones, browsers, social platforms, school portals, job portals, or simple sharing tools.
Review tracks after conversion
Play the MP4 after conversion. Check the selected audio track, subtitles, duration, aspect ratio, and whether the video opens in the destination app before deleting the MKV source.