[Bug 1809479] [NEW] cannot upgrade from 16.04 to 18.04

[huit]

*** This bug is a duplicate of bug 1611737 ***
    https://bugs.launchpad.net/bugs/1611737

Public bug reported:

i just run " sudo apt install update"
when finished, i reboot the pc.Then the system pops a window to upgrade.
so i confirm to do it.
and failed

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.4.0-141.167-generic 4.4.162
Uname: Linux 4.4.0-141-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sat Dec 22 00:00:15 2018
InstallationDate: Installed on 2017-04-09 (621 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-12-21 (0 days ago)
VarLogDistupgradeTermlog:

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug dist-upgrade third-party-packages xenial xenial2bionic

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https://bugs.launchpad.net/bugs/1809479

Title:
  cannot upgrade from 16.04 to 18.04

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  i just run " sudo apt install update"
  when finished, i reboot the pc.Then the system pops a window to upgrade.
  so i confirm to do it.
  and failed

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: ubuntu-release-upgrader-core 1:16.04.26
  ProcVersionSignature: Ubuntu 4.4.0-141.167-generic 4.4.162
  Uname: Linux 4.4.0-141-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.18
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: Unity
  Date: Sat Dec 22 00:00:15 2018
  InstallationDate: Installed on 2017-04-09 (621 days ago)
  InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to xenial on 2018-12-21 (0 days ago)
  VarLogDistupgradeTermlog:

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