unable to upgrade ubuntu18.04.4 to ubuntu20.04

[Armaan]

whenever I am trying to upgrade system, notification pop-up with "could not calculate the upgrade
an unresolvable problem occurred while calculating the upgrade".

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 4.15.0-101.102-generic 4.15.18
Uname: Linux 4.15.0-101-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sun Jun 7 09:32:13 2020
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-xenial-amd64-20160624-2
InstallationDate: Installed on 2018-11-14 (570 days ago)
InstallationMedia: Ubuntu 16.04 "Xenial" - Build amd64 LIVE Binary 20160624-10:47
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-06-07 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2020-06-06T08:40:05.595006