sudo apt-get upgrade error:
dpkg: error: failed to open package info file '/var/lib/dpkg/available' for reading: No such file or directory
E: Sub-process dpkg --set-selections returned an error code (2)
E: Couldn't clean the currently selected dpkg states

sudo apt-get update error.
W: The repository 'http://mirrors.digitalocean.com/ubuntu stretch Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.digitalocean.com/ubuntu stretch-updates Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.digitalocean.com/ubuntu stretch-backports Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://security.ubuntu.com/ubuntu stretch-security Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://repo.mysql.com/apt/debian wheezy InRelease: The following signatures were invalid: EXPKEYSIG 8C718D3B5072E1F5 MySQL Release Engineering <mysql-build@oss.oracle.com>
W: The repository 'http://repo.mysql.com/apt/debian wheezy InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/stretch/main/source/Sources  404  Not Found [IP: 192.241.164.26 80]
E: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/stretch-updates/main/source/Sources  404  Not Found [IP: 192.241.164.26 80]
E: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/stretch-backports/multiverse/source/Sources  404  Not Found [IP: 192.241.164.26 80]
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/stretch-security/multiverse/source/Sources  404  Not Found [IP: 91.189.88.149 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.