apt update

Ign:1 http://mirrors.digitalocean.com/ubuntu stretch InRelease Ign:2 http://mirrors.digitalocean.com/ubuntu stretch-updates InRelease Ign:3 http://mirrors.digitalocean.com/ubuntu stretch-backports InRelease Err:4 http://mirrors.digitalocean.com/ubuntu stretch Release 404 Not Found [IP: 104.24.117.209 80] Err:5 http://mirrors.digitalocean.com/ubuntu stretch-updates Release 404 Not Found [IP: 104.24.117.209 80] Err:6 http://mirrors.digitalocean.com/ubuntu stretch-backports Release 404 Not Found [IP: 104.24.117.209 80] Ign:7 http://security.ubuntu.com/ubuntu stretch-security InRelease Err:8 http://security.ubuntu.com/ubuntu stretch-security Release 404 Not Found [IP: 91.189.91.23 80] Reading package lists… Done E: The repository ‘http://mirrors.digitalocean.com/ubuntu stretch 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://mirrors.digitalocean.com/ubuntu stretch-updates 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://mirrors.digitalocean.com/ubuntu stretch-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://security.ubuntu.com/ubuntu stretch-security 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.