root@debian9:~# sudo apt-get update
-bash: sudo: command not found
root@debian9:~# apt-get update
Get:1 http://ftp.debian.org/debian stretch-backports InRelease [99.9 kB]
Get:2 http://deb.debian.org/debian stable InRelease [116 kB]
Get:3 http://apt.postgresql.org/pub/repos/apt stretch-pgdg InRelease [91.5 kB]
Get:4 http://deb.debian.org/debian stable-updates InRelease [44.1 kB]
Err:1 http://ftp.debian.org/debian stretch-backports InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D1         38 NO_PUBKEY 0E98404D386FA1D9
Err:2 http://deb.debian.org/debian stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D1         38 NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY 605C66F00D6C9793
Ign:5 http://deb.debian.org/debian-security stable/updates InRelease
Err:6 http://deb.debian.org/debian-security stable/updates Release
  404  Not Found [IP: 199.232.138.132 80]
Err:4 http://deb.debian.org/debian stable-updates InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D1         38 NO_PUBKEY 0E98404D386FA1D9
Ign:3 http://apt.postgresql.org/pub/repos/apt stretch-pgdg InRelease
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index file         s will be used. GPG error: http://ftp.debian.org/debian stretch-backports InRelease: The following signatures cou         ldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
W: GPG error: http://deb.debian.org/debian stable InRelease: The following signatures couldn't be verified becaus         e the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY 605C66F00D6C97         93
E: The repository 'http://deb.debian.org/debian stable InRelease' is no longer signed.
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://deb.debian.org/debian-security stable/updates Release' does no longer have a Release fi         le.
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.
W: An error occurred during the signature verification. The repository is not updated and the previous index file         s will be used. GPG error: http://deb.debian.org/debian stable-updates InRelease: The following signatures couldn         't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
W: GPG error: http://apt.postgresql.org/pub/repos/apt stretch-pgdg InRelease: The following signatures couldn't b         e verified because the public key is not available: NO_PUBKEY 7FCC7D46ACCC4CF8
W: The repository 'http://apt.postgresql.org/pub/repos/apt stretch-pgdg 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.
root@debian9:~#