Purged all ppas but cannot do-release-upgrade from Xubuntu 16.04 to 18.04

[kursataker]

Description: Ubuntu 16.04.7 LTS
Release: 16.04

I wanted to upgrade from Xubuntu 16.04 to 18.04. I purged all PPAs.

Still kept receiving this message when sudo do-release-upgrade:

...

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.

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 1:16.04.32
ProcVersionSignature: Ubuntu 4.15.0-24.26~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-24-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.30
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: XFCE
Date: Sat Oct 30 02:04:05 2021
InstallationDate: Installed on 2016-04-01 (2037 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2021-10-29 (0 days ago)