After reading a lot of articles, I heard SELinux is a great way to tighten the security. So I decided to install SELinux and I used below command:

pranav@inspiron-5548:~$ sudo apt-get install selinux

[sudo] password for pranav: 

Sorry, try again.

[sudo] password for pranav: 

Sorry, try again.

[sudo] password for pranav: 

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following additional packages will be installed:

  policycoreutils selinux-policy-dummy

  selinux-utils

The following packages will be REMOVED:

  apparmor gnome-software-plugin-snap snapd

The following NEW packages will be installed:

  policycoreutils selinux selinux-policy-dummy

  selinux-utils

0 upgraded, 4 newly installed, 3 to remove and 0 not upgraded.

Need to get 544 kB of archives.

After this operation, 65.1 MB disk space will be freed.

Do you want to continue? [Y/n] y

Get:1 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 selinux-utils amd64 2.8-1build1 [81.9 kB]

Get:2 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 policycoreutils amd64 2.8-1 [450 kB]

Get:3 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 selinux all 1:0.11 [11.2 kB]

Get:4 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 selinux-policy-dummy all 0.1 [1,730 B]

Fetched 544 kB in 3s (211 kB/s)                  

Preconfiguring packages ...

(Reading database ... 193413 files and directories currently installed.)

Removing gnome-software-plugin-snap (3.30.2-0ubuntu8) ...

Removing snapd (2.37.1+18.10) ...

Removing apparmor (2.12-4ubuntu8) ...

Selecting previously unselected package selinux-utils.

(Reading database ... 193322 files and directories currently installed.)

Preparing to unpack .../selinux-utils_2.8-1build1_amd64.deb ...

Unpacking selinux-utils (2.8-1build1) ...

Selecting previously unselected package policycoreutils.

Preparing to unpack .../policycoreutils_2.8-1_amd64.deb ...

Unpacking policycoreutils (2.8-1) ...

Selecting previously unselected package selinux.

Preparing to unpack .../selinux_1%3a0.11_all.deb ...

Unpacking selinux (1:0.11) ...

Selecting previously unselected package selinux-policy-dummy.

Preparing to unpack .../selinux-policy-dummy_0.1_all.deb ...

Unpacking selinux-policy-dummy (0.1) ...

Processing triggers for mime-support (3.60ubuntu1) ...

Processing triggers for desktop-file-utils (0.23-3ubuntu3) ...

Setting up selinux-utils (2.8-1build1) ...

Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu1) ...

Rebuilding /usr/share/applications/bamf-2.index...

Setting up selinux-policy-dummy (0.1) ...

Processing triggers for systemd (239-7ubuntu10.8) ...

Processing triggers for man-db (2.8.4-2) ...

Processing triggers for gnome-menus (3.13.3-11ubuntu2) ...

Setting up policycoreutils (2.8-1) ...

selinux-autorelabel-mark.service is a disabled or a static unit, not starting it.

Setting up selinux (1:0.11) ...

Sourcing file `/etc/default/grub'

Generating grub configuration file ...

Found linux image: /boot/vmlinuz-4.18.0-16-generic

Found initrd image: /boot/initrd.img-4.18.0-16-generic

Found linux image: /boot/vmlinuz-4.18.0-15-generic

Found initrd image: /boot/initrd.img-4.18.0-15-generic

Found Windows Boot Manager on /dev/sda2@/EFI/Microsoft/Boot/bootmgfw.efi

Adding boot menu entry for EFI firmware configuration

done

update-rc.d: warning: start and stop actions are no longer supported; falling back to defaults

Processing triggers for systemd (239-7ubuntu10.8) ...

Processing triggers for initramfs-tools (0.131ubuntu15.1) ...

update-initramfs: Generating /boot/initrd.img-4.18.0-16-generic

And then restarted my computer. The following images appear:

First thing that appears

Second image

Note that this machine is dual booted with Windows 10.

After adding selinux=0 in the grub menu, I was able to gain back my PC running.

What am I missing here? What should I do? Should I uninstall SELinux or manage it to work properly?

SELinux is not fully supported in Ubuntu as they focus on apparmor and as already said by other people, it will probably require to adjust the policy.

But if you want to give it a try, you need to install an actual policy, here I see that you installed selinux-policy-dummy which is described as:

Empty Security-Enhanced Linux policy (dummy package)

This package is used to satisfy package dependencies, but does not
contain an actual SELinux policy. This package should be installed
if the system administrator is developing a custom SELinux policy.

You probably want to install selinux-policy-default instead, this package contains an actual policy, the system should boot a bit further. You also might want to check in /etc/selinux/config that the mode is set to permissive, that will make SELinux only log denials.

Edit: Also, try to install selinux-basic instead of selinuxpackage, the later is ubuntu specific and not maintained for years