update failed from 16.04 to 18.04

[Alex Zhang]

正在检查软件包管理器
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正在计算变更

正在计算变更

无法计算升级

An unresolvable problem occurred while calculating the upgrade.

This was likely caused by:
* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

如果这些都不生效，请在终端中使用命令 'ubuntu-bug ubuntu-release-upgrader-core' 来报告
bug。If you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.

正在恢复原始系统状态

中止
正在读取软件包列表... 完成
正在分析软件包的依赖关系树
正在读取状态信息... 完成

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.30
ProcVersionSignature: Ubuntu 4.15.0-65.74~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-65-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Wed May 20 13:15:30 2020
InstallationDate: Installed on 2019-01-16 (490 days ago)
InstallationMedia: Ubuntu 16.04.5 LTS "Xenial Xerus" - Release amd64 (20180731)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2020-05-20 (0 days ago)