could not calculate upgrade while upgrading to 18.04

[Andrew Wang]

While trying to upgrade to Ubuntu 18.04, I get this error message:

Could not calculate the upgrade

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.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.29
ProcVersionSignature: Ubuntu 4.4.0-177.207-generic 4.4.214
Uname: Linux 4.4.0-177-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Apr 19 11:29:04 2020
InstallationDate: Installed on 2018-06-17 (672 days ago)
InstallationMedia: Ubuntu-Server 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.8)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2020-04-19 (0 days ago)
VarLogDistupgradeTermlog: