the update from 18.04 to 20.04 doesn't works properly

[simone merlotti]

sudo do-release-upgrade -d -f DistUpgradeViewGtk3 doesn't terminate the insstallation

1st advise
Third party sources disabled

Some third party entries in your sources.list were disabled. You can re-enable them after the upgrade with the 'software-properties' tool or your package manager.

I chose Close

Could not calculate the upgrade

then appear
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 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-48.52~18.04.1-generic 5.4.60
Uname: Linux 5.4.0-48-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
Date: Wed Sep 30 21:22:51 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-09-30 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2020-09-30 21:21:45.721706
 Log time: 2020-09-30 21:22:08.161679
 Log time: 2020-09-30 21:22:37.723017
 Log time: 2020-09-30 21:22:56.863454
VarLogDistupgradeTermlog: