Debian 6.0 Jessie issue with 404 Errors during apt-get update + Webmin dependencies with no installation candidate FIX
These are commands executed and outputs. Aim was to make apt-get update running and then installing Webmin.

I was successfull so im sharing there:

# cat /etc/issue
Debian GNU/Linux 6.0 \n \l
dpkg --install webmin_1.801_all.deb

dpkg: dependency problems prevent configuration of webmin:
webmin depends on libnet-ssleay-perl; however:
Package libnet-ssleay-perl is not installed.
webmin depends on libauthen-pam-perl; however:
Package libauthen-pam-perl is not installed.
webmin depends on libio-pty-perl; however:
Package libio-pty-perl is not installed.
webmin depends on apt-show-versions; however:
Package apt-show-versions is not installed.
dpkg: error processing webmin (--install):
dependency problems - leaving unconfigured
Errors were encountered while processing:
webmin

apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl apt-show-versions python

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libnet-ssleay-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package libauthen-pam-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package libio-pty-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package apt-show-versions is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'libnet-ssleay-perl' has no installation candidate
E: Package 'libauthen-pam-perl' has no installation candidate
E: Package 'libio-pty-perl' has no installation candidate
E: Package 'apt-show-versions' has no installation candidate
apt-get update

...
Err http://ftp.debian.org squeeze/main Sources
404 Not Found
...
W: Failed to fetch http://ftp.debian.org/debian/dists/squeeze/main/source/Sources.gz 404 Not Found
Do update /etc/apt/sources.list from https://debgen.simplylinux.ch/generate.php

In my case i used this content into sources.list:

deb http://ftp.us.debian.org/debian/ jessie main contrib non-free
deb-src http://ftp.us.debian.org/debian/ jessie main contrib non-free
deb http://download.webmin.com/download/repository sarge contrib

Comment out all other lines adding # to the beginning of old lines

apt-get update

W: GPG error: http://download.webmin.com sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51
W: There is no public key available for the following key IDs:
CBF8D6FD518E17E1
gpg --keyserver pgpkeys.mit.edu --recv-key D97A3AE911F63C51
gpg -a --export D97A3AE911F63C51 | sudo apt-key add -
apt-get update

W: There is no public key available for the following key IDs:
CBF8D6FD518E17E1
gpg --keyserver pgpkeys.mit.edu --recv-key CBF8D6FD518E17E1
gpg -a --export CBF8D6FD518E17E1 | sudo apt-key add -
apt-get update
apt-get upgrade

NOW WEBMIN:

wget -q http://www.webmin.com/jcameron-key.asc -O- | apt-key add -

apt-get install webmin