If you run tuna without a subcommand it tries to run a gui. If you are installed on a server without a display you get some kind of an error message. It would be better to detect the situation and exit gracefully with some kind of an explanation or usage message. You get the same result when you issue the gui subcommand
su -c "tuna gui" Password:(tuna:7645): Gtk-CRITICAL **: 11:31:55.573: _gtk_css_lookup_resolve: assertion '(((__extension__ ({ GTypeInstance *__inst = (GTypeInstance*) ((provider)); GType __t = ((_gtk_style_provider_private_get_type ())); gboolean __r; if (!__inst) __r = (0); else if (__inst->g_class && __inst->g_class->g_type == __t) __r = (!(0)); else __r = g_type_check_instance_is_a (__inst, __t); __r; }))))' failed /usr/lib/python3.9/site-packages/tuna/tuna_gui.py:40: Warning: g_object_set_data_full: assertion 'G_IS_OBJECT (object)' failed self.wtree.add_objects_from_file(tuna_glade, ("mainbig_window", "tuna"))(tuna:7645): Gtk-ERROR **: 11:31:55.573: Can't create a GtkStyleContext without a display connection Trace/breakpoint trap (core dumped) [acme@nine linux]$