Cannot join an external screen and the screen of the labtop

I can use either the labtop screen OR the DELL external screen but I cannot join the two screens, as it was possible (and very easy) with Ubuntu 16.04. It did not work with 18.04, it does not work with 20.04. It is very frustrating since it works when I start the computer, I can see the "Ubuntu" word on both the screens, but after I enter my password I can use only one of the screen (I change from "parameters").

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: x11-session-utils 7.7+4
ProcVersionSignature: Ubuntu 5.4.0-48.52-generic 5.4.60
Uname: Linux 5.4.0-48-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.9
Architecture: amd64
BootLog: Error: [Errno 13] Permission non accordée: '/var/log/boot.log'
CasperMD5CheckResult: skip
CompositorRunning: None
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 9 15:20:09 2020
DistUpgraded: Fresh install
DistroCodename: focal
DistroVariant: ubuntu
GraphicsCard:
 Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller [8086:2a42] (rev 07) (prog-if 00 [VGA controller])
   Subsystem: Dell Mobile 4 Series Chipset Integrated Graphics Controller [1028:024d]
   Subsystem: Dell Mobile 4 Series Chipset Integrated Graphics Controller [1028:024d]
InstallationDate: Installed on 2020-10-09 (0 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
MachineType: Dell Inc. Latitude E4300
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-48-generic root=UUID=bb032965-73b6-4d9f-b40a-5f7813dbb097 ro quiet splash
SourcePackage: x11-session-utils
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/22/2009
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A14
dmi.board.name: 0D201R
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA14:bd12/22/2009:svnDellInc.:pnLatitudeE4300:pvr:rvnDellInc.:rn0D201R:rvr:cvnDellInc.:ct8:cvr:
dmi.product.name: Latitude E4300
dmi.sys.vendor: Dell Inc.
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.101-2
version.libgl1-mesa-dri: libgl1-mesa-dri 20.0.8-0ubuntu1~20.04.1
version.libgl1-mesa-glx: libgl1-mesa-glx N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.20.8-2ubuntu2.4
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:19.1.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20200226-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.16-1