Can't install required updates

[Arello]

I've been trying to update my Ubuntu Mate 16.04 after a two month trip. I cannot proceed with any update. Even though the manager recommends partial update to do something, that also gets an error message:

Can't install 'ubuntu-desktop'

It was impossible to install a required package. Please report this as a bug using 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

So, I followed the orders and here I am. I am not sure if I should have ticked this as a security vulnerability, as it prevents doing security updates altogether. (I have never reported bugs.)

'apt-cache policy pkgname' cannot find any package named pkgname, but Software Center says it's 15.12.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.21
ProcVersionSignature: Ubuntu 4.4.0-79.100-generic 4.4.67
Uname: Linux 4.4.0-79-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: MATE
Date: Sun Aug 20 12:04:26 2017
InstallationDate: Installed on 2016-10-04 (319 days ago)
InstallationMedia: Ubuntu-MATE 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeTermlog: