@jommy
If you are using Roku Media Player (RMP), I have found that sometimes just changing the "format container" solves many of the "file not playable" errors.
I primarily use ffmpeg and use the following command at the command prompt to convert mp4 (and other formats) to mkv...
ffmpeg -i b.mp4 -c:v copy -c:a copy b.mkv
[In the above command, "b" is the video file I am converting. I just use "b" rather than using the title of the video file to keep the commands simple. Then after the file is converted, I rename "b" to the desired video title.] The above command simply converts the file from mp4 to mkv without transcoding so very quick. (seconds).
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Others find transcoding using Handbrake more user-friendly than ffmpeg, but then need to wait while Handbrake goes through the longer process of transcoding to a format RMP likes better.
Still others like using other server software like Servioo, Plex, and Emby for playing their media files on the Roku since it takes care of the transcoding on the fly. I actually like using Jellyfin and Kodi when the need arises and I happen to have those computers/servers on. Some of the above have channels/apps in the Roku Channel Store that makes their use pretty user-friendly (after a small learning curve).
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I am assuming that you are using the compatible formats of RMP. https://support.roku.com/article/208754908
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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