upgrade to 20.04 LTS crashes

[r c]

I am trying to update to 20.04 and i get this error message :

Calculating the changes

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.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.40
ProcVersionSignature: Ubuntu 5.4.0-52.57~18.04.1-generic 5.4.65
Uname: Linux 5.4.0-52-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Oct 22 17:18:03 2020
InstallationDate: Installed on 2020-01-12 (284 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-22 (0 days ago)