unable to connect to server

[johnsereno]

the first couple of parts seem to work OK. But when it is downloading "new" system it gives a notice that it cannot connect to a valid server. I know my Internet connection is OK and I am able to download upgrades to 16;04 OK but am not able to upgrade to 18.04. Please let me know how I should proceed. Thank you.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.4.0-134.160-generic 4.4.140
Uname: Linux 4.4.0-134-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
Date: Mon Sep 10 15:50:31 2018
InstallationDate: Installed on 2016-03-18 (906 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-09-10 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2018-09-10 15:49:02.641547
 Log time: 2018-09-10 15:49:13.706049
 Log time: 2018-09-10 15:49:55.513035
 Log time: 2018-09-10 15:50:33.460097
VarLogDistupgradeTermlog: