can not do a release upgrade from 18.04 to 20.04

[Hans Winter]

when runnuning
do-release-upgrade -d -f DistUpgradeViewGtk3

upgrade installation fails

1) lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04

2) do-release-upgrade -d -f DistUpgradeViewGtk3

3) I expect getting upgraded from Ubuntu 18.04 to 20.04

4) Distribution upgrade fails:
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 1:18.04.40
ProcVersionSignature: Ubuntu 5.4.0-54.60~18.04.1-generic 5.4.65
Uname: Linux 5.4.0-54-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.20
Architecture: amd64
CrashDB: ubuntu
Date: Sat Nov 28 08:33:13 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-11-28 (0 days ago)
VarLogDistupgradeTermlog: