A while ago, Valve added a built-in system for creating and sharing your
gameplay footage in the Steam Client. This makes it easy to record game
videos "on the fly" and easily share clips without having to learn OBS
Studio and video editing software. Game recording works quite well and
was even ported to the Steam for Linux Client.

There is one glitch, however. On Linux, with a AMD GPU, and sometimes
with an Nvidia GPU, trying to export a video or video clip from the 
'Original' tab results in  the error:

"Failed to export, try again"

Not very helpful.  This occurs when trying to export in H.265 format -
the default format Steam uses to record the video. Turns out this was
reported a while back:

https://github.com/ValveSoftware/steam-for-linux/issues/11850

Steam lis looking for libcuda.so.1, even when running on a non-Nvidia 
GPU. You can export with transcoding to H.264 in the 'Custom' tab just
fine.  This works (you may need to install ffmpeg and the relevent
codecs for your linux distro.

I found a workaround in a small blog post:

https://y.tsutsumi.io/reading-steam-game-recordings

To copy the original H.265 stream, look in you Steam dirctory:

.local/share/Steam/userdata/{id}/gamerecordings/clips

You will see one or more directories of the format:

clip_{game id}_{date_time in UTC}

Locate your game and open the directory. The original vidoe is the in

video/fg_{game id}_{date_time in UTC}

where you with find the video stream *.m4s files as well as the important
'session.mpd' XML file.  Ffmpeg can read mpd file natively, so simply

ffmpeg -i session.mpd -c copy out.mp4

Other container formats compatible with H.265 and AAC audio will work\
as well. You now have the full original video to copy, upload, modify etc.

Have Fun!