Can't Install R 4.X on Ubuntu 20.04

I trying to update R on my Ubuntu 20.04 (new updated from 16.04).

So I delete the previous version of R by :

sudo apt-get purge r-base* r-recommended r-cran-*    
sudo apt autoremove

After, I update my /etc/apt/sources.list by adding the good repository

deb https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/

And the correct key :

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9

But, I obtain an error during the update of sources.list :

sudo apt-get update
Ign :10 https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ InRelease
Err :11 https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ Release
  Could not wait for server fd - select (11: Ressource temporairement non disponible) [IP : xxx.xxx.xxx.xxx xxxx]
Lecture des listes de paquets... Fait
W: Le fichier configuré « partener/source/Sources » ne sera pas pris en compte car le dépôt « http://archive.canonical.com/ubuntu focal InRelease » ne dispose pas de la source « partener » (erreur de saisie dans sources.list ?)
E: Le dépôt https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ Release n'a pas de fichier Release.

At work, I am behind a proxy. So, the proxy parameters are specified on the network parameters in systeme parameters but also for apt. (80proxy file in /etc/apt/apt.conf.d/)

With this configuration, only the R 3.6 is accessible :

apt-cache policy r-base r-base-core r-base-dev r-recommended
r-base:
  Installé : (aucun)
  Candidat : 3.6.3-2
 Table de version :
     3.6.3-2 500
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe i386 Packages
r-base-core:
  Installé : (aucun)
  Candidat : 3.6.3-2
 Table de version :
     3.6.3-2 500
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
r-base-dev:
  Installé : (aucun)
  Candidat : 3.6.3-2
 Table de version :
     3.6.3-2 500
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe i386 Packages
r-recommended:
  Installé : (aucun)
  Candidat : 3.6.3-2
 Table de version :
     3.6.3-2 500
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        500 http://fr.archive.ubuntu.com/ubuntu focal/universe i386 Packages

I have got a laptop with also Ubuntu 20.04 updated and the same sources.list and I meet no error from home network.

I suspect that the error is due to proxy parameters but I not sure. Could you help to configure and install successfully the last version of R ?

Thanks a lot for your help.

My sources.list :

 more /etc/apt/sources.list
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://fr.archive.ubuntu.com/ubuntu/ focal main restricted
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://fr.archive.ubuntu.com/ubuntu/ focal-updates main restricted
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://fr.archive.ubuntu.com/ubuntu/ focal universe
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal universe
deb http://fr.archive.ubuntu.com/ubuntu/ focal-updates universe
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://fr.archive.ubuntu.com/ubuntu/ focal multiverse
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal multiverse
deb http://fr.archive.ubuntu.com/ubuntu/ focal-updates multiverse
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://fr.archive.ubuntu.com/ubuntu/ focal-backports main restricted univers
e multiverse
deb-src http://fr.archive.ubuntu.com/ubuntu/ focal-backports main restricted uni
verse multiverse

deb http://security.ubuntu.com/ubuntu focal-security main restricted
deb-src http://security.ubuntu.com/ubuntu focal-security main restricted
deb http://security.ubuntu.com/ubuntu focal-security universe
deb-src http://security.ubuntu.com/ubuntu focal-security universe
deb http://security.ubuntu.com/ubuntu focal-security multiverse
deb-src http://security.ubuntu.com/ubuntu focal-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu focal partner
deb-src http://archive.canonical.com/ubuntu focal partner

## The Comprehensive R Archive Network

deb https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/
# deb-src https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/

After contacting the IT service, they ask to me to test another address for the proxy parameter but I obtain a new error.

sudo apt-get update
Ign :1 https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ InRelease
Atteint :2 http://fr.archive.ubuntu.com/ubuntu focal InRelease                                                                            
Atteint :3 http://fr.archive.ubuntu.com/ubuntu focal-updates InRelease                                                                                   
Err :4 https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ Release                                                                                
  Could not handshake: An unexpected TLS packet was received. [IP : 10.3.0.11 3128]
Atteint :5 http://fr.archive.ubuntu.com/ubuntu focal-backports InRelease                                                                                 
Atteint :6 http://repository.spotify.com stable InRelease                                                                                 
Atteint :7 http://ppa.launchpad.net/c2d4u.team/c2d4u4.0+/ubuntu focal InRelease                         
Réception de :8 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Lecture des listes de paquets... Fait
E: Le dépôt https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ Release n'a pas de fichier Release.
N: Les mises à jour depuis un tel dépôt ne peuvent s'effectuer de manière sécurisée, et sont donc désactivées par défaut.
N: Voir les pages de manuel d'apt-secure(8) pour la création des dépôts et les détails de configuration d'un utilisateur.

With this new error, I found this previous post

apt update: Could not handshake: An unexpected TLS packet was received

So I try to delete the "s" in the address of https_proxy and.... it's WORKS !!!

In addition, the only proxy parameters take account in the apt-get update command are in the /etc/apt/apt.conf file.

more apt.conf
Acquire::http::proxy "http://myaddress.XXX.fr:myport/";
Acquire::https::proxy "http://myaddress.XXX.fr:myport/";
Acquire::ftp::proxy "ftp://myaddress.XXX.fr:myport/";

This solution works also with the initial proxy parameters.