Similar to this https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1787720

[aquannie]

After checking main.log I saw debug log of foreign for certain packages. Then I unchecked their PPAs from the Other Software tab in Settings. Some packages stopped coming in the log. For the rest I just tried to for example do

sudo apt install cpp-4.9
Reading package lists... Done
Building dependency tree
Reading state information... Done
cpp-4.9 is already the newest version (4.9.4-2ubuntu1~16.04).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

This removed the foreign error but gave new errors in the main.log

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.24
Uname: Linux 4.13.3-041303-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Dec 27 13:38:18 2019
InstallationDate: Installed on 2017-11-01 (785 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-12-27 (0 days ago)
VarLogDistupgradeTermlog:

[aquannie (aquannie)]

The exact error message was

Could not calculate the upgrade

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'.