apt-get error message

The issue is that Debian 8 is no longer providing jessie-updates, so that line needs to be commented out in the appropriate file.

First, run the following command:

sudo nano /etc/apt/sources.list

The following lines should be in that file:

deb http://mirrors.linode.com/ jessie/updates main
deb-src http://mirrors.linode.com/ jessie/updates main

deb http://mirrors.linode.com/debian jessie-updates main
deb-src http://mirrors.linode.com/debian jessie-updates main

What you'll want to do is put a '#' before each of those lines, which is how you comment them out. Once that's done, save and exit.

Here's a Stack Exchange article that gives a good explanation:

https://unix.stackexchange.com/questions/508724/failed-to-fetch-jessie-backports-repository/508728#508728