Attempted to upgrade from 18.04 to 20.04 - unresolvable error

[Sam Cook]

I was attempting to upgrade from version 18.04 to 20.04 and was met with this error message.

Could not determine 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'.

System Details
Description: Ubuntu 18.04.5 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 5.4.0-66.74~18.04.2-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Feb 25 18:04:17 2021
InstallationDate: Installed on 2020-05-28 (273 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-02-25 (0 days ago)
VarLogDistupgradeTermlog: