I try to install Let’s Encrypt on my Ubuntu machine but get this error:

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://archive.ubuntu.com/ubuntu cosmic InRelease
Ign:2 http://security.ubuntu.com/ubuntu cosmic-security InRelease
Hit:3 http://ppa.launchpad.net/certbot/certbot/ubuntu cosmic InRelease
Err:4 http://archive.ubuntu.com/ubuntu cosmic Release
  404  Not Found [IP: 91.189.88.152 80]
Err:5 http://security.ubuntu.com/ubuntu cosmic-security Release
  404  Not Found [IP: 91.189.88.142 80]
Hit:6 https://packages.microsoft.com/ubuntu/18.04/prod bionic InRelease
Ign:7 http://mirrors.digitalocean.com/ubuntu cosmic InRelease
Ign:8 http://mirrors.digitalocean.com/ubuntu cosmic-updates InRelease
Ign:9 http://mirrors.digitalocean.com/ubuntu cosmic-backports InRelease
Err:10 http://mirrors.digitalocean.com/ubuntu cosmic Release
  404  Not Found [IP: 104.24.117.209 80]
Err:11 http://mirrors.digitalocean.com/ubuntu cosmic-updates Release
  404  Not Found [IP: 104.24.117.209 80]
Err:12 http://mirrors.digitalocean.com/ubuntu cosmic-backports Release
  404  Not Found [IP: 104.24.117.209 80]
Reading package lists... Done
E: The repository 'http://archive.ubuntu.com/ubuntu cosmic 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://security.ubuntu.com/ubuntu cosmic-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.
E: The repository 'http://mirrors.digitalocean.com/ubuntu cosmic 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://mirrors.digitalocean.com/ubuntu cosmic-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://mirrors.digitalocean.com/ubuntu cosmic-backports 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.