can't upgrade to ubuntu 20

[Ze'ev Dreifuss]

Could not calculate 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'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 5.4.0-62.70~18.04.1-generic 5.4.78
Uname: Linux 5.4.0-62-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Wed Jan 20 08:35:09 2021
InstallationDate: Installed on 2020-06-07 (226 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-01-20 (0 days ago)
VarLogDistupgradeTermlog: