I encountered the same error (see duplicate https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/1930974/comments/5) and was able to resolve the issue by reinstalling packages "python3-six" and "python3-certifi" as suggested in https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/1900098/comments/2. Those packages were installed before.

The relevant journalctl section is:

May 18 18:45:47 gungnir dbus-daemon[1746]: [system] Activating service name='com.ubuntu.SoftwareProperties' requested by ':1.7781' (uid=1000 pid=497419 comm="/usr/bin/python3 /usr/bin/software-properties-gtk " label="unconfined") (using servicehelper)
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Unable to init server: Could not connect: Connection refused
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Unable to init server: Could not connect: Connection refused
May 18 18:45:48 gungnir dbus-daemon[1746]: [system] Successfully activated service 'com.ubuntu.SoftwareProperties'
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Traceback (most recent call last):
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/usr/lib/software-properties/software-properties-dbus", line 68, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     server = SoftwarePropertiesDBus(bus, datadir=datadir)
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/softwareproperties/dbus/SoftwarePropertiesDBus.py", line 66, in __init__
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     self._livepatch_service = LivepatchService()
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/softwareproperties/LivepatchService.py", line 93, in __init__
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     self._session = requests_unixsocket.Session()
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: NameError: name 'requests_unixsocket' is not defined
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Error in sys.excepthook:
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Traceback (most recent call last):
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/apport_python_hook.py", line 72, in apport_excepthook
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     from apport.fileutils import likely_packaged, get_recent_crashes
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     from apport.report import Report
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/apport/report.py", line 32, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     import apport.fileutils
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/apport/fileutils.py", line 12, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     import os, glob, subprocess, os.path, time, pwd, sys, requests_unixsocket
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/requests_unixsocket/__init__.py", line 1, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     import requests
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/requests/__init__.py", line 112, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     from . import utils
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/requests/utils.py", line 24, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     from . import certs
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/requests/certs.py", line 15, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     from certifi import where
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: ModuleNotFoundError: No module named 'certifi'
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Original exception was:
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: Traceback (most recent call last):
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/usr/lib/software-properties/software-properties-dbus", line 68, in <module>
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     server = SoftwarePropertiesDBus(bus, datadir=datadir)
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/softwareproperties/dbus/SoftwarePropertiesDBus.py", line 66, in __init__
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     self._livepatch_service = LivepatchService()
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:   File "/lib/python3/dist-packages/softwareproperties/LivepatchService.py", line 93, in __init__
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]:     self._session = requests_unixsocket.Session()
May 18 18:45:48 gungnir com.ubuntu.SoftwareProperties[497432]: NameError: name 'requests_unixsocket' is not defined
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]: ERROR:dbus.proxies:Introspect error on :1.7782:/: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NoReply: Message recipient disconnected from message bus without replying
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]: Traceback (most recent call last):
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:   File "/usr/bin/software-properties-gtk", line 100, in <module>
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:     app = SoftwarePropertiesGtk(datadir=options.data_dir, options=options, file=file)
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:   File "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 222, in __init__
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:     self.backend.Reload();
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:   File "/usr/lib/python3/dist-packages/dbus/proxies.py", line 72, in __call__
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:     return self._proxy_method(*args, **keywords)
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:   File "/usr/lib/python3/dist-packages/dbus/proxies.py", line 141, in __call__
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:     return self._connection.call_blocking(self._named_service,
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:   File "/usr/lib/python3/dist-packages/dbus/connection.py", line 652, in call_blocking
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]:     reply_message = self.send_message_with_reply_and_block(
May 18 18:45:48 gungnir software-properties-drivers.desktop[497419]: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name :1.7782 was not provided by any .service files
May 18 18:45:48 gungnir systemd[1]: Starting Process error reports when automatic reporting is enabled...
May 18 18:45:48 gungnir systemd[2437]: gnome-launched-software-properties-drivers.desktop-497419.scope: Succeeded.