Error upgrading distro

[Simon George]

Trying to update system from 18.04 to 20.04 get error

An unresolvable problem occurred while calculating the upgrade.

This was likely caused by:
* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

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'.

There are no errors shown in main.log

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
Uname: Linux 4.15.0-117-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sun Sep 20 14:57:04 2020
InstallationDate: Installed on 2017-01-11 (1348 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: dist-upgrade
UpgradeStatus: Upgraded to bionic on 2020-09-20 (0 days ago)

[Daniel Letzeisen (dtl131)]

"There are no errors shown in main.log"
Look again:
2020-09-20 14:56:32,417 ERROR Dist-upgrade failed: 'E:Unable to correct problems, you have held broken packages.'

This is caused by having 3rd party PPA's and repositories installed (look at your /etc/apt/sources.list.d directory). It looks like the oibaf PPA specifically is causing the issue, so:
sudo apt-get install ppa-purge
sudo ppa-purge ppa:oibaf/graphics-drivers

I recommend you do the same with other bionic PPA's you have installed, and also remove the old .list files (ones from xenial and debian stable).

After you clean up your sources, try upgrading again. Please let us know whether upgrade succeeds or fails.

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]