libopengl0 on 20.04 cannot be installed

What is the output of:

lsb_release -a; uname -a; apt-cache policy libopengl0 libglvnd0

Thanks

The package version in the default repos are at the right versions
https://packages.ubuntu.com/search?keywords=libglvnd0&searchon=names&suite=focal&section=all
https://packages.ubuntu.com/search?suite=focal&section=all&arch=any&keywords=libopengl0&searchon=names

and will install

lsb_release -a; uname -a; apt-cache policy libopengl0 libglvnd0

No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.1 LTS (fossa-bulbasaur X38)
Release: 20.04
Codename: focal
Linux ...... 5.6.0-1033-oem #35-Ubuntu SMP Thu Oct 22 10:55:50 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
libopengl0:
  Installed: (none)
  Candidate: 1.3.1-1
  Version table:
     1.3.1-1 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
libglvnd0:
  Installed: 1.3.1-1ubuntu0.20.04.1
  Candidate: 1.3.1-1ubuntu0.20.04.1
  Version table:
 *** 1.3.1-1ubuntu0.20.04.1 100
        100 /var/lib/dpkg/status
     1.3.1-1 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages

Technically, libglvnd0 is holding it back, somehow I have the update version, although not really understanding, which repo is giving it to me

$ sudo apt update

Hit:1 http://ppa.launchpad.net/fenics-packages/fenics/ubuntu focal InRelease
Hit:2 http://oem.archive.canonical.com focal InRelease
Hit:3 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:4 http://dell.archive.canonical.com focal InRelease
Hit:5 http://ppa.launchpad.net/somerville-dla-team/ppa/ubuntu bionic InRelease
Hit:6 https://packages.microsoft.com/repos/ms-teams stable InRelease
Hit:7 https://apt.syncthing.net syncthing InRelease
Hit:8 http://archive.canonical.com/ubuntu focal InRelease
Hit:9 http://us.archive.ubuntu.com/ubuntu focal InRelease
Get:10 http://us.archive.ubuntu.com/ubuntu focal-security InRelease [109 kB]
Get:11 http://us.archive.ubuntu.com/ubuntu focal-security/main amd64 DEP-11 Metadata [24,3 kB]
Get:12 http://us.archive.ubuntu.com/ubuntu focal-security/universe amd64 DEP-11 Metadata [56,5 kB]
Fetched 190 kB in 3s (62,2 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done

$ sudo apt install libglvnd0
Reading package lists... Done
Building dependency tree
Reading state information... Done
libglvnd0 is already the newest version (1.3.1-1ubuntu0.20.04.1).
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

$ sudo apt install --reinstall libglvnd0
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reinstallation of libglvnd0 is not possible, it cannot be downloaded.

I could use aptitude and downgrade the following packages

     Downgrade the following packages:
1) libegl1 [1.3.1-1ubuntu0.20.04.1 (now) -> 1.3.1-1 (focal)]
2) libgl1 [1.3.1-1ubuntu0.20.04.1 (now) -> 1.3.1-1 (focal)]
3) libgles2 [1.3.1-1ubuntu0.20.04.1 (now) -> 1.3.1-1 (focal)]
4) libglvnd0 [1.3.1-1ubuntu0.20.04.1 (now) -> 1.3.1-1 (focal)]
5) libglx0 [1.3.1-1ubuntu0.20.04.1 (now) -> 1.3.1-1 (focal)]

then libopengl0 could be installed and all is working now!

You already identified the root cause: you do not have focal-updates enabled in your list of repositories.

Open the software-properties-gtk program and enable it, or add the lines the /etc/apt/sources.list.