What is error message when running command in the CLI?
Please see the last line
root@ubuntu18:/etc/nginx/sites-available# sudo add-apt-repository ppa:certbot/certbot
This is the PPA for packages prepared by Debian Let's Encrypt Team and backported for Ubuntu.
Note: Packages are only provided for currently supported Ubuntu releases.
More info: https://launchpad.net/~certbot/+archive/ubuntu/certbot
Press [ENTER] to continue or Ctrl-c to cancel adding it.
Ign:1 http://ppa.launchpad.net/certbot/certbot/ubuntu eoan InRelease
Get:2 http://security.ubuntu.com/ubuntu eoan-security InRelease [97.5 kB]
Hit:3 http://archive.ubuntu.com/ubuntu eoan InRelease
Get:4 http://archive.ubuntu.com/ubuntu eoan-updates InRelease [97.5 kB]
Err:5 http://ppa.launchpad.net/certbot/certbot/ubuntu eoan Release
404 Not Found [IP: 91.189.95.83 80]