Cannot install updates

[Clara Welch]

It seems you have modified the contents of "/etc/update-manager/release-upgrades". Would you like to add the contents of it to your bug report?

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.23
Uname: Linux 4.13.0-041300rc6-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Thu Feb 15 19:32:36 2018
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-xenial-amd64-20160624-2
InstallationDate: Installed on 2017-05-23 (268 days ago)
InstallationMedia: Ubuntu 16.04 "Xenial" - Build amd64 LIVE Binary 20160624-10:47
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2016-06-24T06:39:06