I am unable to upgrade 18.04 to 20.04

[Hel Bahadur Thapa]

It is failed at preparing for upgrade phase.

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.

Invalid package information
After updating your package information, the essential package 'ubuntu-minimal' could not be located. This may be because you have no official mirrors listed in your software sources, or because of excessive load on the mirror you are using. See /etc/apt/sources.list for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade again later.

Thanx & regards
HEl

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 5.3.0-51.44~18.04.2-generic 5.3.18
Uname: Linux 5.3.0-51-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CrashDB: ubuntu
Date: Wed Apr 29 13:04:05 2020
InstallationDate: Installed on 2020-01-03 (117 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-04-29 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2020-04-29 13:03:25.125378
 Log time: 2020-04-29 13:03:39.459447
 Log time: 2020-04-29 13:04:03.059866
 Log time: 2020-04-29 13:04:11.263255
VarLogDistupgradeTermlog: