I have tried multiple times to upgrade from 18.04 to 20.04 but it fails everytime

[hiren]

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'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.44
ProcVersionSignature: Ubuntu 4.15.0-51.55-generic 4.15.18
Uname: Linux 4.15.0-51-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.24
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Wed Jul 14 12:24:46 2021
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-07-14 (0 days ago)

[Chris Guiver (guiverc)]

Thank you for taking the time to report this bug and helping to make Ubuntu better.

I see a number of 3rd party packages, esp. from https://launchpad.net/~oibaf/+ppa-packages

For best results with a release-upgrade, you should remove all 3rd party packages (bringing your system back to a pure Ubuntu one, ie. no 3rd party packages) then perform the upgrade. The tool ppa-purge can assist with this.

Likely related to https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1069133

(following from https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1922546/comments/2)

[The use of oibaf ppa] will prevent the upgrade as that PPA contains package version numbers greater than the release to which you are upgrading. You can revert the PPA by installing the ppa-purge package from the official Ubuntu archive and then running 'sudo ppa-purge ppa:oibaf/graphics-drivers'. After that you can try upgrading again and if it still fails open a new bug report. Thanks and good luck!