can't upgrade to 18.04

[loai]

could not calculate the upgrade

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.15.0-39.42~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-39-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Tue Nov 27 21:20:48 2018
InstallationDate: Installed on 2018-10-21 (36 days ago)
InstallationMedia: Ubuntu 16.04.4 LTS "Xenial Xerus" - Release amd64 (20180228)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-11-27 (0 days ago)
VarLogDistupgradeTermlog:

modified.conffile..etc.update-manager.release-upgrades: [modified]
mtime.conffile..etc.update-manager.release-upgrades: 2018-11-17T19:31:14.573337