I’ve got the same error using the Wayland version of KDE plasma on linux mint. (Not on the X11 version though). I got it working using the normal brave-browser rather than the flatpak. Haven’t tried the SNAP but it might work.
Not much help on what is actually going wrong though.
Solved I think (at least for flatpak if you are on wayland) . Try running flatpak run com.brave.Browser --enable-features=UseOzonePlatform --ozone-platform=wayland
If that works then edit com.brave.Browser.desktop replacing instances of EXEC=… with EXEC=flatpak run com.brave.Browser --enable-features=UseOzonePlatform --ozone-platform=wayland