je souhaite passer de la version 16-04 LTS à la 18-04 LTS

[philippe logeais]

Impossible d'évaluer la mise à niveau

An unresolvable problem occurred while calculating the upgrade.

Si rien de tout cela ne s'applique, veuillez signaler ce bogue en
utilisant la commande « ubuntu-bug ubuntu-release-upgrader-core »
dans un 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 16.04
Package: ubuntu-release-upgrader-core 1:16.04.32
ProcVersionSignature: Ubuntu 4.4.0-210.242-generic 4.4.262
Uname: Linux 4.4.0-210-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.30
Architecture: amd64
CrashDB: ubuntu
Date: Fri Jun 18 15:15:31 2021
InstallationDate: Installed on 2014-03-02 (2664 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20120425-15:28
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2021-06-18 (0 days ago)