Comment 0 for bug 1887544

the problem is during issue this command from terminal to start upgrade process to 20.04 from 18.04 using this command
 sudo do-release-upgrade -d

it makes this output
sudo do-release-upgrade -d
[sudo] password for amr:
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1,554 B]
Get:2 Upgrade tool [1,342 kB]
Fetched 1,343 kB in 6s (211 kB/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
extracting 'focal.tar.gz'

Reading cache

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Hit http://archive.ubuntu.com/ubuntu bionic InRelease
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:3 http://archive.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:4 http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages [783 kB]
Get:5 http://archive.ubuntu.com/ubuntu bionic-security/main i386 Packages [506 kB]
Get:6 http://archive.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [679 kB]
Get:7 http://archive.ubuntu.com/ubuntu bionic-security/universe i386 Packages [625 kB]
Fetched 2,845 kB in 6s (15.3 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done

Checking for installed snaps

Calculating snap size requirements

Updating repository information
Hit http://archive.ubuntu.com/ubuntu focal InRelease
Hit http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit http://archive.ubuntu.com/ubuntu focal-backports InRelease
Hit http://archive.ubuntu.com/ubuntu focal-security InRelease
Fetched 0 B in 6s (0 B/s)

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Calculating the changes

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.
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'.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 5.3.0-62.56~18.04.1-generic 5.3.18
Uname: Linux 5.3.0-62-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Tue Jul 14 17:08:10 2020
InstallationDate: Installed on 2020-02-27 (137 days ago)
InstallationMedia: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-07-14 (0 days ago)