it could not investigated which system-upgrades are available

[Reinhard]

first step :
starting tool ppa purge, having swept off all none official packages

second step
 starting upgrade routine for 20.04 lts upgrade

third step
got notice " it could not investigated which system upgrade are availiable

update procedure stopped

well, give me a solution for this problem

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.40
ProcVersionSignature: Ubuntu 4.15.0-122.124-generic 4.15.18
Uname: Linux 4.15.0-122-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Tue Nov 10 00:16:08 2020
InstallationDate: Installed on 2018-08-20 (812 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-11-09 (0 days ago)
VarLogDistupgradeTermlog: