Release upgrade 18.4->20.4 fails

[Garry]

After update/upgrade to all most current packages under 18.04, rebooted, then tried "do-release-upgrade"

root@dck:~# lsb_release -rd
Description: Ubuntu 18.04.4 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 4.15.0-99.100-generic 4.15.18
Uname: Linux 4.15.0-99-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CrashDB: ubuntu
Date: Sun May 10 14:10:04 2020
InstallationDate: Installed on 2016-07-17 (1393 days ago)
InstallationMedia: Ubuntu-Server 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.3)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-05-10 (0 days ago)

[Brian Murray (brian-murray)]

You have a version of nodejs installed which has a version number greater than the one from the official Ubuntu archive which is preventing the upgrade from being calculated. You have nodejs
12.16.3-1nodesource1 while the our releases contain the following versions:

 $ rmadison nodejs
 nodejs | 0.6.12~dfsg1-1ubuntu1 | precise/universe | source, amd64, armel, armhf, i386
 nodejs | 0.10.25~dfsg2-2ubuntu1 | trusty/universe | source, amd64, armhf, i386, powerpc, ppc64el
 nodejs | 0.10.25~dfsg2-2ubuntu1.2 | trusty-security/universe | source, amd64, armhf, i386, powerpc, ppc64el
 nodejs | 0.10.25~dfsg2-2ubuntu1.2 | trusty-updates/universe | source, amd64, armhf, i386, powerpc, ppc64el
 nodejs | 4.2.6~dfsg-1ubuntu4 | xenial/universe | source, amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 nodejs | 4.2.6~dfsg-1ubuntu4.2 | xenial-security/universe | source, amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 nodejs | 4.2.6~dfsg-1ubuntu4.2 | xenial-updates/universe | source, amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 nodejs | 8.10.0~dfsg-2 | bionic/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 nodejs | 8.10.0~dfsg-2ubuntu0.2 | bionic-security/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 nodejs | 8.10.0~dfsg-2ubuntu0.4 | bionic-updates/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 nodejs | 10.15.2~dfsg-1 | disco/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 nodejs | 10.15.2~dfsg-2ubuntu1 | eoan/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 nodejs | 10.19.0~dfsg-3ubuntu1 | focal/universe | source, amd64, arm64, armhf, ppc64el, s390x

You'll need to install the version of nodejs from the official archive of Ubuntu for the upgrade to be calculable.