upgrading from 18.04.4 to 20.04 fails

[Andrew Santhosh]

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.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 4.15.0-109.110-generic 4.15.18
Uname: Linux 4.15.0-109-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
Date: Tue Jun 30 00:48:46 2020
InstallationDate: Installed on 2019-10-18 (255 days ago)
InstallationMedia: Ubuntu 16.04.6 LTS "Xenial Xerus" - Release amd64 (20190227)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_IN:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_IN
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-06-29 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

I suspect you have some packages leftover from a previous release of Ubuntu which require python (as opposed to python3) installed on your system. Could you run the command 'ubuntu-support-status --list | grep Unsupported' and add the output to this bug report? Thanks in advance.

[Andrew Santhosh (andrew264)]

I reinstalled Ubuntu 20.04 using bootable disk and that fixed it. Thanks