Could not calculate the upgrade

I want to upgrade my Ubuntu 18.04 LTS to 20.04.2 LTS, but got this:

Preparing to upgrade=> Done
Setting new software channels=>
-------------------------------download=> Done
-------------------------------calculating the upgrade=> error

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 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'.

When I go to the terminal and use the command:

shashwat@shashwat-Inspiron-3521:~$ ubuntu-bug ubuntu-release-upgrader-core
comm: /var/log/dmesg: No such file or directory

This is the second error I encounter.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 4.15.0-140.144-generic 4.15.18
Uname: Linux 4.15.0-140-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Fri Apr 2 11:16:25 2021
InstallationDate: Installed on 2019-01-07 (815 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-04-02 (0 days ago)
VarLogDistupgradeTermlog: