It says unresolvable problem occured while calculating the upgrade (from ubuntu 18.04 to 20.04).

[Sadman Sakib Riham]

It occurred while upgrading from Lubuntu 18.04.05 LTS to Lubuntu 20.04 LTS

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If you want to investigate this yourself the log files in '/var/log/dist-upgrade' will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.

I have only installed Steam, Pycharm Community, Python 3.8, Google Chrome, and Zoom on this PC.

My specs:
Intel(R) Pentium(R) Dual CPU E2180 @ 2.00GHz 1 physical processor; 2 threads; 2 cores
Intel Family G33/G31 graphics
RAM: 2028964KiB (2GB)
Ubuntu Distro Flavor: Lubuntu
Ubuntu Version: 18.04.05 LTS

I am a newb to Ubuntu. So please help me with a simple solution. I have been waiting for this update for so long. ( I don't know how to use ppa-purge and what it is )

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.40
ProcVersionSignature: Ubuntu 4.15.0-122.124-generic 4.15.18
Uname: Linux 4.15.0-122-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Mon Nov 9 23:56:34 2020
InstallationDate: Installed on 2020-04-16 (207 days ago)
InstallationMedia: Lubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-11-09 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2020-11-07T22:22:02.965298