То есть ты наворотил себе незнамо что, начитавшись неясно каких инструкций в инете, а сейчас хочешь, чтобы мы, не зная, ни что это за инструкции, ни что именно ты себе наделал, дали тебе команду, которая всё исправит? А ты забавный.

Вангую, что xserver-xorg-core конфликтует с более полной версией xserver. Проверь, стоит ли у тебя kubuntu-desktop

systemctl get-default хоть бы логи какие приложил.

/var/log/apt/history.log покажите. Подозреваю, что каким-то образом был удалён менеджер входа (sddm).

Вывод systemctl list-unit-files --no-pager также не помешает.

history

Start-Date: 2020-05-17 10:19:00 Commandline: apt install xserver-xorg-core Requested-By: u-a50 (1000) Install: xserver-xorg-core:amd64 (2:1.19.6-1ubuntu4.4) Remove: xserver-xorg-input-libinput-hwe-18.04:amd64 (0.28.1-1~18.04.1), xserver-xorg-video-vmware-hwe-18.04:amd64 (1:13.3.0-2build1~18.04.1), xserver$ End-Date: 2020-05-17 10:19:04

Start-Date: 2020-05-17 10:19:20 Commandline: apt install xserver-xorg-input-synaptics Requested-By: u-a50 (1000) Install: xserver-xorg-input-synaptics:amd64 (1.9.0-1ubuntu1) End-Date: 2020-05-17 10:19:22

Start-Date: 2020-05-17 11:26:00 Commandline: apt autoremove xserver-xorg-core Requested-By: u-a50 (1000) Remove: xserver-xorg-input-synaptics:amd64 (1.9.0-1ubuntu1), xserver-xorg-core:amd64 (2:1.19.6-1ubuntu4.4) End-Date: 2020-05-17 11:26:02

Start-Date: 2020-05-17 11:35:38 Commandline: apt install xserver-xorg-core Requested-By: u-a50 (1000) Install: xserver-xorg-core:amd64 (2:1.19.6-1ubuntu4.4) End-Date: 2020-05-17 11:35:39

Start-Date: 2020-05-17 11:43:50 Commandline: apt install xserver-xorg-input-synaptics Requested-By: u-a50 (1000) Install: xserver-xorg-input-synaptics:amd64 (1.9.0-1ubuntu1) End-Date: 2020-05-17 11:43:52

Start-Date: 2020-05-17 13:31:40 Install: libreoffice-l10n-ru:amd64 (1:6.0.7-0ubuntu0.18.04.10, automatic), libreoffice-help-ru:amd64 (1:6.0.7-0ubuntu0.18.04.10) End-Date: 2020-05-17 13:31:41

Start-Date: 2020-05-17 16:27:40 Commandline: apt install ssh Requested-By: u-a50 (1000) Install: openssh-sftp-server:amd64 (1:7.6p1-4ubuntu0.3, automatic), ssh:amd64 (1:7.6p1-4ubuntu0.3), ncurses-term:amd64 (6.1-1ubuntu1.18.04, automatic$ End-Date: 2020-05-17 16:27:44

Start-Date: 2020-05-17 18:50:05 Commandline: apt autoremove xserver-xorg-core Requested-By: u-a50 (1000) Remove: xserver-xorg-input-synaptics:amd64 (1.9.0-1ubuntu1), xserver-xorg-core:amd64 (2:1.19.6-1ubuntu4.4), xserver-xorg-input-evdev:amd64 (1:2.10.5-$ End-Date: 2020-05-17 18:50:07

Start-Date: 2020-05-17 18:50:39 Commandline: apt install xserver-xorg-core Requested-By: u-a50 (1000) Install: xserver-xorg-core:amd64 (2:1.19.6-1ubuntu4.4) End-Date: 2020-05-17 18:50:41

Start-Date: 2020-05-17 18:55:40 Commandline: apt install xserver-xorg-input-evdev Requested-By: u-a50 (1000) Install: xserver-xorg-input-evdev:amd64 (1:2.10.5-1ubuntu1) End-Date: 2020-05-17 18:55:41

Start-Date: 2020-05-17 18:55:53 Commandline: apt install xserver-xorg-input-synaptics Requested-By: u-a50 (1000) Install: xserver-xorg-input-synaptics:amd64 (1.9.0-1ubuntu1) End-Date: 2020-05-17 18:55:55

systemctl list-unit-files

systemd-ask-password-console.path static
UNIT FILE STATE
proc-sys-fs-binfmt_misc.automount static
-.mount generated
boot-efi.mount generated
boot.mount generated
dev-hugepages.mount static
dev-mqueue.mount static
proc-sys-fs-binfmt_misc.mount static
sys-fs-fuse-connections.mount static
sys-kernel-config.mount static
sys-kernel-debug.mount static
acpid.path enabled
apport-autoreport.path enabled
cups.path enabled
systemd-ask-password-plymouth.path static
systemd-ask-password-wall.path static
session-1.scope transient
accounts-daemon.service enabled
acpid.service disabled
alsa-restore.service static
alsa-state.service static
alsa-utils.service masked
apparmor.service enabled
apport-autoreport.service static
anacron.service enabled
apport-forward@.service static
apport.service generated
apt-daily-upgrade.service static
apt-daily.service static
autovt@.service enabled
avahi-daemon.service enabled
binfmt-support.service enabled
blk-availability.service enabled
bluetooth.service enabled
bootlogd.service masked
bootlogs.service masked
bootmisc.service masked
checkfs.service masked
checkroot-bootclean.service masked
checkroot.service masked
clean-mount-point@.service static
colord.service static
configure-printer@.service static
console-getty.service disabled
console-setup.service enabled
container-getty@.service static
cron.service enabled
cryptdisks-early.service masked
cryptdisks.service masked
cups-browsed.service enabled
cups.service enabled
dbus-fi.w1.wpa_supplicant1.service enabled
dbus-org.bluez.service enabled
dbus-org.freedesktop.Avahi.service enabled
dbus-org.freedesktop.hostname1.service static
dbus-org.freedesktop.locale1.service static
dbus-org.freedesktop.login1.service static
dbus-org.freedesktop.ModemManager1.service enabled
dbus-org.freedesktop.nm-dispatcher.service enabled
dbus-org.freedesktop.resolve1.service enabled
dbus-org.freedesktop.thermald.service enabled
dbus-org.freedesktop.timedate1.service static
dbus.service static
debug-shell.service disabled
display-manager.service generated
dm-event.service static
emergency.service static
friendly-recovery.service static
fstrim.service static
fuse.service masked
fwupd-offline-update.service static
fwupd.service static
geoclue.service static
getty-static.service static
getty@.service enabled
gpu-manager.service enabled

sddm.service generated

sudo dpkg-reconfigure sddm

sudo apt install kubuntu --reinstall

Спасибо. Помогло.