same issue for me, I tried to launch a Plasma session with wayland compositor via SDDM, and it failed and return to SDDM.
> cat ~/.wayland-errors
startplasmacompositor: Starting up... dbus-update-activation-environment: warning: error sending to systemd: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1 /usr/bin/startplasmacompositor: 222: /usr/bin/startplasmacompositor: --xwayland: not found startplasmacompositor: Shutting down... xprop: unable to open display '' xprop: unable to open display '' startplasmacompositor: Done.