Optimize video files for android phone/tablet

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Your media library should ideally match your devices (clients) that you use for playback. This includes video compatibility (codecs, profiles, levels, tiers), audio compatibility (channels, codecs, bitrate, sample rate, bit depth), and subtitle compatibility (format, complexity). This is not always possible, which is why tools like Jellyfin exist, to transcode "on-the-fly" as you watch.

Your media not being supported, or that message more specifically, means that you have something wrong with your configuration or the file you're trying to transcode is not compatible with the hardware you have set up. If you take a look at the NVIDIA Matrix, you can search for your card and you'll see that it's pretty widely compatible, except for certain pixel formats in HEVC and it doesn't support AV1 decoding at all.

This leads me to believe that there's an issue with configuration.

Could you provide more information about your media file? Depending on your OS, you could run mediainfo or ffprobe against the file to get more information. Looking for information such as what's listed above -- codecs, profiles, levels, tiers, channels, bitrates, subtitle info, etc...