Setup X11VNC Server 

-Step 1 – Install X11VNC
sudo apt update
sudo apt install x11vnc

-Step 2 – Create Password
Now create a password to connect using vnc viewer from the client system.
 ##############
--------->  x11vnc -storepasswd 
##############
Enter VNC password: *********
Verify password: *********  
Write password to /home/user/.vnc/passwd?  [y]/n y
Password written to: /home/USER/.vnc/passwd

-Step 3 – Start X11VNC Server
###################
------------->   sudo x11vnc -auth /home/USER/.Xauthority -display :0  -forever -loop -noxdamage -repeat rfbauth /home/USER/.vnc/passwd -rfbport 5900 -shared 
###################
The VNC server will start on default port 5900. In case the port 5900 is busy with some other service, it will automatically select the next available port like 5901.

-Step 4 – Connect from VNC Client
To connect to the server using VNC, you need a VNC client on your system. For this tutorial, I am using RealVNC viewer on our system. You can use any other client of your choice.

setup x11vnc on ubuntu

###############################" ######################  For AUTOSTART :

1. make file x11vnc.service in /etc/systemd/system :
sudo mcedit /etc/systemd/system/x11vnc.service 
with text (change user for you name):

########
[Unit]
Description=x11vnc remote desktop server
After=multi-user.target

[Service]
Type=simple
ExecStart=/usr/bin/x11vnc -auth /home/USER/.Xauthority -display :0  -forever -loop -noxdamage -repeat rfbauth /home/USER/.vnc/passwd -rfbport 5900 -shared
Restart=on-failure

[Install]
WantedBy=multi-user.target
###########

2. sudo systemctl daemon-reload
sudo systemctl start x11vnc
sudo systemctl status x11vnc

3. if it’s worked
sudo systemctl enable x11vnc.service

####
#######################
Probleme Permission d'accès au serveur X
x11vnc -auth /home/nom_User/.Xauthority -display :0
x11vnc -auth /tmp/.gdmzndVlR -display :0
#########
##
#alias vnc='x11vnc -auth /home/ubuntu40/.Xauthority -display :0'
#alias vnc='x11vnc -rfbauth /home/ubuntu40/.vnc/passwd -auth /home/ubuntu40/.Xauthority -display :0'
alias vnc='x11vnc -auth /home/ubuntu40/.Xauthority -display :0'