If you get the error message

  org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.Notifications was not provided by any .service files

on starting up systray-mdstat, your window manager, desktop
environment. .xsession or .xinitrc doesn't seem to start up the
installed notification-daemon.

You then might need to add it manually to the list of automatically
started daemons upon user login or add an according line to your
.xsession or .xinitrc like e.g. this one:

  /usr/lib/notification-daemon/notification-daemon &

See https://bugs.debian.org/974949 for a more in-depth discussion.

 -- Axel Beckert <abe@debian.org>, Sat, 26 Dec 2020 08:07:05 +0100