can not update to 20.04 LTS.

[Jigar Parekh]

I tried all the things here-
https://askubuntu.com/questions/1238620/i-was-unable-to-update-to-ubuntu-20-from-ubuntu-18-04

But it still gives the same error -
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was caused by:
 * Upgrading to a pre-release version of Ubuntu
This is most likely a transient problem,
please try again later.
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 also attached the 'main.log' file.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.40
ProcVersionSignature: Ubuntu 4.15.0-123.126-generic 4.15.18
Uname: Linux 4.15.0-123-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Wed Nov 11 14:57:50 2020
InstallationDate: Installed on 2019-01-23 (657 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-11-11 (0 days ago)
VarLogDistupgradeAptclonesystemstate.tar.gz: Error: command ['pkexec', 'cat', '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'] failed with exit code 127: Error checking for authorization org.freedesktop.policykit.exec: GDBus.Error:org.freedesktop.DBus.Error.NoReply: Message recipient disconnected from message bus without replying
VarLogDistupgradeTermlog: