Can't Upgrade from 16.04 to 18.04, no details in error message

[Jonathan Jiang]

I am upgrading from 16.04 to 18.04. I removed all non-ubuntu packages because that cause complaints at first.

Now there is an error, but it doesn't specify why it is failing.

I only get:
"
An unresolvable problem occurred while calculating the upgrade.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.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'.
"

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.29
ProcVersionSignature: Ubuntu 4.15.0-96.97~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-96-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Apr 24 13:05:15 2020
InstallationDate: Installed on 2016-09-12 (1320 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2020-04-24 (0 days ago)