The unofficial Steam Flatpak app requires a correctly-configured desktop
session, which must provide the DISPLAY environment variable to the
D-Bus session bus activation environment.
On systems that use systemd --user, the DISPLAY environment variable must
also be present in the systemd --user activation environment.
This is usually achieved by running:
dbus-update-activation-environment DISPLAY
during desktop environment startup.
For more details, please see:
https://github.com/ValveSoftware/steam-for-linux/issues/10554 how to fix this issue
