The client isn't compatible..

[TheDreadPirate]

Whatever repo you got this from is not using the latest build. And it is using an especially old version of ffmpeg. Not that the RPi can do any kind of transcoding at a useful resolution or frame rate, which is what your setup is trying to do.

Whatever client you are using does not support your media directly, which is why Jellyfin is trying to transcode.

Code:

TranscodeReason=ContainerNotSupported, VideoCodecNotSupported, AudioCodecNotSupported