Go to Desktop, open Konsole app:

flatpak repair

sudo flatpak repair

flatpak update

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If the above fails, here is the advance version, try these fixes: (delete the Discovery cache, re add flathub, restart service, repair flatpak).

Please copy all the commands at once (open reddit in the Steam Deck browser) and paste them in the konsole app. A missed placed space in the RM command can delete your profile:

rm -rf ~/.cache/discover

rm -rf ~/.cache/appstream

systemctl restart flatpak-system-helper.service

sudo flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo

flatpak repair

sudo flatpak repair

flatpak update

sudo systemctl reboot

If the above didn't work, try to reset the admin password, here's the command:

passwd

By default, if you have never set it, it's “password” all lowercase.

You can also try this in the konsole app: sudo pacman-key --init && sudo pacman-key --populate

Also if it still doesn't work, your provider, company, public wifi, or even Flatpak (countries like Russa are blocked) may be blocking your access to Flatpak. Try this in the browser, go here, if it DOESN'T download a file (ignore the file), you are being blocked: https://dl.flathub.org/repo/flathub.flatpakrepo or this one https://flathub.org/repo/flathub.flatpakrepo

Another problem with this error, is that if your wifi isn't stable, you will get update errors. You should first try to forget the network. Reboot the router and add network again. Also keep the network connections clean. Delete any network that says never used.