ununderstood data member

For diagnostic purposes please provide the output that you receive for the commands

uname -a
lsb_release -crid
apt-cache policy dpkg apt synaptic
sudo apt update
sudo dpkg --audit

Whatever you did, it did not work. Please copy and paste the output as plain text into https://answers.launchpad.net/ubuntu/+source/dpkg/+question/693344

Linux rigelanon 4.4.0-173-generic #203-Ubuntu SMP Wed Jan 15 02:55:01 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

sudo lsb_release -crid
Distributor ID: LinuxMint
Description: Linux Mint 18.3 Sylvia
Release: 18.3
Codename: sylvia

apt-cache policy dpkg apt synaptic
dpkg:
Installed: 1.18.4ubuntu1.6
Candidate: 1.18.4ubuntu1.6
Version table: *** 1.18.4ubuntu1.6 500 500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages 100 /var/lib/dpkg/status 1.18.4ubuntu1 500 500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages

apt: Installed: 1.2.32ubuntu0.1 Candidate: 1.2.32ubuntu0.1 Version table: *** 1.2.32ubuntu0.1 500 500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages 500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages 100 /var/lib/dpkg/status 1.2.10ubuntu1 500 500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages

synaptic: Installed: 0.83linuxmint4 Candidate: 0.83linuxmint4 Version table: *** 0.83linuxmint4 700 700 http://packages.linuxmint.com sylvia/upstream amd64 Packages 100 /var/lib/dpkg/status 0.83 500 500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages

sudo apt update Ign:1 http://packages.linuxmint.com sylvia InRelease Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB] Hit:3 http://archive.canonical.com/ubuntu xenial InRelease Get:4 http://debian.fhem.de/nightly InRelease [1 912 B] Hit:5 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu xenial InRelease Hit:6 http://archive.ubuntu.com/ubuntu xenial InRelease Get:7 http://packages.microsoft.com/repos/vscode stable InRelease [3 958 B] Ign:8 http://live.linuxmint.com sarah InRelease Hit:9 http://packages.linuxmint.com sylvia Release Get:11 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 Packages [757 kB] Get:12 http://debian.fhem.de/nightly Packages [770 B] Hit:13 http://ppa.launchpad.net/openmw/openmw/ubuntu xenial InRelease Get:14 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB] Get:15 http://packages.microsoft.com/repos/vscode stable/main amd64 Packages [203 kB] Hit:16 http://live.linuxmint.com sarah Release Hit:18 http://ppa.launchpad.net/openscad/releases/ubuntu xenial InRelease Get:19 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB] Hit:20 http://ppa.launchpad.net/vcmi/ppa/ubuntu xenial InRelease Get:21 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages [1 861 kB] Get:22 http://security.ubuntu.com/ubuntu xenial-security/universe i386 Packages [638 kB] Hit:23 http://ppa.launchpad.net/viktornova/random/ubuntu xenial InRelease Hit:24 http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu xenial InRelease Get:25 http://archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages [1 435 kB] Get:26 http://security.ubuntu.com/ubuntu xenial-security/universe Translation-en [214 kB] Get:27 http://archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages [1 180 kB] Get:28 http://archive.ubuntu.com/ubuntu xenial-updates/universe i386 Packages [1 050 kB] Get:29 http://archive.ubuntu.com/ubuntu xenial-updates/universe Translation-en [345 kB] Fetched 8 014 kB in 17s (453 kB/s) Reading package lists... Done Building dependency tree Reading state information... Done 43 packages can be upgraded. Run 'apt list --upgradable' to see them.

sudo apt update
Hit:1 http://archive.canonical.com/ubuntu xenial InRelease
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB]
Ign:3 http://packages.linuxmint.com sylvia InRelease
Hit:4 http://packages.linuxmint.com sylvia Release
Hit:6 http://archive.ubuntu.com/ubuntu xenial InRelease
Get:7 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
Hit:8 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu xenial InRelease
Ign:9 http://live.linuxmint.com sarah InRelease
Hit:10 http://debian.fhem.de/nightly InRelease
Get:11 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
Hit:12 http://ppa.launchpad.net/openmw/openmw/ubuntu xenial InRelease
Hit:13 http://packages.microsoft.com/repos/vscode stable InRelease
Hit:14 http://live.linuxmint.com sarah Release
Get:16 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages [1 861 kB]
Hit:17 http://ppa.launchpad.net/openscad/releases/ubuntu xenial InRelease
Hit:18 http://ppa.launchpad.net/vcmi/ppa/ubuntu xenial InRelease
Get:19 http://archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages [1 435 kB]
Hit:20 http://ppa.launchpad.net/viktornova/random/ubuntu xenial InRelease
Hit:21 http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu xenial InRelease
Fetched 3 621 kB in 14s (255 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
43 packages can be upgraded. Run 'apt list --upgradable' to see them.

sudo dpkg --audit
Feeds CR to new command prompt

Michael

Manfred Hampl suggests this article as an answer to your question:
FAQ #3041: “No support for Linux Mint”.