sudo apt-get update has 4 error on Ubuntu Server

I am new to Ubuntu Server configuration and I’m using 19.4 version but when I try to do sudo apt-get update I have these errors:

Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease
Ign:2 http://archive.ubuntu.com/ubuntu disco-updates InRelease
Ign:3 http://archive.ubuntu.com/ubuntu disco InRelease
Ign:4 http://archive.ubuntu.com/ubuntu disco-backports InRelease
Ign:5 http://archive.ubuntu.com/ubuntu disco-security InRelease
Err:6 http://archive.ubuntu.com/ubuntu disco-updates Release
  404  Not Found (IP: 91.189.88.142 80)
Err:7 http://archive.ubuntu.com/ubuntu disco Release
  404  Not Found (IP: 91.189.88.142 80)
Err:8 http://archive.ubuntu.com/ubuntu disco-backports Release
  404  Not Found (IP: 91.189.88.142 80)
Err:9 http://archive.ubuntu.com/ubuntu disco-security Release
  404  Not Found (IP: 91.189.88.142 80)
Reading package lists... Done
E: The repository 'http://archive.ubuntu.com/ubuntu disco-updates Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. 
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. 
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco-backports Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. 
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco-security Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. 
N: See apt-secure(8) manpage for repository creation and user configuration details.

I had to get ssh with sudo apt-get install ssh. I couldn’t do that neither due to same error so I did vim /etc/apt/sources.list and changed to

deb http://archive.ubuntu.com/ubuntu disco main restricted 

to

deb http://archive.ubuntu.com/ubuntu focal main restricted 

because my professor said so. With this change I could set up ssh and status is active but still I can’t get any update. I ping to 8.8.8.8 and it works also. Further I have to do

sudo apt-get install isc-dhcp-server 

and inside I have to change INTERFACESv4="enp0s8". But I couldn’t do because of these errors as well.

I have these addresses in /etc/netplan

network:
  version: 2
  renderer: networkd
  ethernets:
      enp0s3:
           dhcp4: yes
           dhcp6: yes
      enp0s8:
           dhcp4: no
           dhcp6: no
           addresses: (10.1.1.1/24)
      enp0s9:
           dhcp4: no
           dhcp6: no
           addresses: (192.168.56.100/24)

I hope I’m clear!

Many thanks