Comment 3 for bug 2012288

Update, I found that CUPS was failing to be modified by the gnome(?) user interface component.

apparmor is getting in the way, "protecting" cupsd.

To fix this, install the apparmor utilities:

sudo apt-get install apparmor-utils

Then set apparmor to complain about cupsd, instead of blocking access:

sudo aa-complain cupsd

You can now print. I hope this helps someone else too. This was quite difficult to troubleshoot.

ar 24 16:10:45 semiauto google-chrome.desktop[216920]: [216913:216943:0324/161045.967690:ERROR:connection_factory_impl.cc(428)] Failed to connect to MCS endpoint with error -105
Mar 24 16:10:46 semiauto google-chrome.desktop[216920]: [216913:216913:0324/161046.442656:ERROR:device_event_log_impl.cc(222)] [16:10:46.442] Printer: local_printer_handler_default.cc:167 Failure enumerating local printers, result: kFailed
Mar 24 16:10:46 semiauto google-chrome.desktop[216920]: Warning: disabling flag --expose_wasm due to conflicting flags
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:325: DeprecationWarning: Gtk.ActionGroup.list_actions is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: for action in printer_manager_action_group.list_actions ():
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:326: DeprecationWarning: Gtk.Action.set_sensitive is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: action.set_sensitive (False)
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:334: DeprecationWarning: Gtk.ActionGroup.get_action is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: act = printer_manager_action_group.get_action (action)
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:364: DeprecationWarning: Gtk.UIManager.ensure_update is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: self.ui_manager.ensure_update ()
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:365: DeprecationWarning: Gtk.UIManager.get_accel_group is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: self.PrintersWindow.add_accel_group (self.ui_manager.get_accel_group ())
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:374: DeprecationWarning: Gtk.UIManager.get_action is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: action = self.ui_manager.get_action ("/new-printer")
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:375: DeprecationWarning: Gtk.Action.create_menu_item is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: newprinteritem = action.create_menu_item ()
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:673: DeprecationWarning: Gtk.Action.get_proxies is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: for widget in action.get_proxies ():
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: /usr/share/system-config-printer/system-config-printer.py:2253: DeprecationWarning: Gdk.threads_enter is deprecated
Mar 24 16:10:51 semiauto google-chrome.desktop[216920]: Gdk.threads_enter ()
Mar 24 16:10:51 semiauto dbus-daemon[764]: [system] Activating via systemd: service name='org.freedesktop.hostname1' unit='dbus-org.freedesktop.hostname1.service' requested by ':1.1686' (uid=1000 pid=395280 comm="/usr/bin/python3 /usr/share/system-config-printer/" label="unconfined")
Mar 24 16:10:51 semiauto systemd[1]: Starting Hostname Service...
Mar 24 16:10:51 semiauto dbus-daemon[764]: [system] Successfully activated service 'org.freedesktop.hostname1'
Mar 24 16:10:51 semiauto systemd[1]: Started Hostname Service.
Mar 24 16:10:53 semiauto kernel: [663246.142511] serial 0000:00:16.3: LSR safety check engaged!
Mar 24 16:10:53 semiauto kernel: [663246.142539] audit: type=1400 audit(1679688653.733:100): apparmor="DENIED" operation="capable" class="cap" profile="/usr/sbin/cupsd" pid=395323 comm="serial" capability=21 capname="sys_admin"

Here were a few other telling messages:
Mar 24 16:13:14 semiauto kernel: [663387.182005] audit: type=1400 audit(1679688794.769:101): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:17 semiauto kernel: [663389.718118] audit: type=1400 audit(1679688797.305:102): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:24 semiauto kernel: [663397.180664] audit: type=1400 audit(1679688804.769:103): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:24 semiauto kernel: [663397.180678] audit: type=1400 audit(1679688804.769:104): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:26 semiauto kernel: [663399.385467] audit: type=1400 audit(1679688806.973:105): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:26 semiauto kernel: [663399.385511] audit: type=1400 audit(1679688806.973:106): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:29 semiauto kernel: [663401.903042] audit: type=1400 audit(1679688809.489:107): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0
Mar 24 16:13:31 semiauto kernel: [663403.494875] audit: type=1400 audit(1679688811.081:108): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0

Mar 24 16:14:50 semiauto kernel: [663483.050466] audit: type=1400 audit(1679688890.638:111): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0

Mar 24 16:14:52 semiauto kernel: [663485.093715] audit: type=1400 audit(1679688892.682:112): apparmor="DENIED" operation="file_lock" class="file" profile="/usr/sbin/cupsd" name="/run/utmp" pid=287961 comm="cupsd" requested_mask="k" denied_mask="k" fsuid=0 ouid=0