mate upgrade 18.10 to 20.04 could not calculate the upgrade

[Michael]

Upgrading mate in virtualbox vm (vb version 6.1.10r138449), received "Could no calculate the upgrade"
Likely caused by indicates unofficial software packages not provided by ubuntu.
I removed the wireguard ppa, this cleared the error.

Then proceeded with upgrade again and received another error likely caused by:
Upgrading to a pre-release version of Ubuntu

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 5.3.0-62.56~18.04.1-generic 5.3.18
Uname: Linux 5.3.0-62-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: MATE
Date: Sun Jul 12 22:56:23 2020
InstallationDate: Installed on 2020-01-29 (165 days ago)
InstallationMedia: Ubuntu-MATE 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-07-13 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

There is a new version of the release upgrader available. Could you please try using that and see if it resolves the failure to calculate the upgrade?

[Michael (armsmi)]

Thank you, I was able to successfully upgrade to 20.04.

[Brian Murray (brian-murray)]

Great, I'm glad you were able to successfully upgrade. Enjoy 20.04!