Cannot install docker on Ubuntu 20.04 LTS

I'm new using docker and linux and I tried to install docker on my new Ubuntu 20.04 LTS.

I was following these instructions: https://www.digitalocean.com/community/tutorials/how-to-install-and-use-docker-on-ubuntu-20-04

When I arrived at this command: sudo apt install docker-ce, I got this error:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
docker-ce is already the newest version (5:19.03.9~3-0~ubuntu-focal).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Setting up docker-ce (5:19.03.9~3-0~ubuntu-focal) ...
Job for docker.service failed because the control process exited with error code.
See "systemctl status docker.service" and "journalctl -xe" for details.
invoke-rc.d: initscript docker, action "start" failed.
● docker.service - Docker Application Container Engine
     Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
     Active: activating (auto-restart) (Result: exit-code) since Sat 2020-05-23 22:00:38 CES
T; 8ms ago
TriggeredBy: ● docker.socket
       Docs: https://docs.docker.com
    Process: 15674 ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containe
rd.sock (code=exited, status=1/FAILURE)
   Main PID: 15674 (code=exited, status=1/FAILURE)
dpkg: error processing package docker-ce (--configure):
 installed docker-ce package post-installation script subprocess returned error exit status 
1
Errors were encountered while processing:
 docker-ce
E: Sub-process /usr/bin/dpkg returned an error code (1)

Can someone help me to resolve this ?

For learning purposes, I would like to know what is going wrong and why. But isn't mandatory as long as this works again.

Thank you very much.