Unable to lock screen in elementary OS Juno

Question

I am unable to lock my desktop in Elementary OS, juno. I had earlier installed GNOME desktop and when I tried to remove it, I was left with no desktop environment. I then installed xfce desktop environment, followed by pantheon. Although I am able to login into a pantheon environment, I am unable to remove xfce or ubuntu completely and have a pure elementary desktop experience. The lock screen also does not work. Using the command 'windows+L' returns nothing, neither does locking screen via the console.

My xsessions returns the following:

ls -l /usr/share/xsessions

total 20
-rw-r--r-- 1 root root  183 Sep 14  2018 pantheon.desktop
-rw-r--r-- 1 root root  323 May  2  2018 ubuntu-communitheme-snap.desktop
-rw-r--r-- 1 root root  247 May  2  2018 ubuntu.desktop
-rw-r--r-- 1 root root 5465 Sep 21  2016 xfce.desktop

Output for

grep -r --include '*.list' '^deb ' /etc/apt/ | sed -re 's/^\/etc\/apt\/sources\.list((\.d\/)?|(:)?)//' -e 's/(.*\.list):/\[\1\] /' -e 's/deb http:\/\/ppa.launchpad.net\/(.*?)\/ubuntu .*/ppa:\1/'

[miktex.list] deb http://miktex.org/download/ubuntu xenial universe
[elementary.list] ppa:elementary-os/stable
[windscribe-repo.list] deb https://repo.windscribe.com/ubuntu zesty main
[appcenter.list] deb http://packages.elementary.io/appcenter bionic main
[patches.list] ppa:elementary-os/os-patches
[spotify.list] deb http://repository.spotify.com stable non-free
[ubuntuhandbook1-ubuntu-audacity-bionic.list] ppa:ubuntuhandbook1/audacity
deb http://us.archive.ubuntu.com/ubuntu/ bionic main restricted
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
deb http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
deb https://dl.winehq.org/wine-builds/ubuntu/ cosmic main

Answer 1

Please execute the series of commands from the terminal,

sudo apt-get purge xfce4
sudo apt-get purge ubuntu-desktop
sudo apt-get autoremove
sudo snap remove communitheme
sudo apt-get autoremove
sudo apt-get clean
sudo apt-get autoclean
sudo apt-get -f install
sudo apt-get update
sudo apt-get upgrade

Here we are removing xfce desktop, ubuntu desktop, community theme, cleaning the system and making an update & upgrade after that.