It looks like either your sources.list file is either missing, or else it has been edited. This is the file that Apt, and associated programs, use to determine from where to download packages. By default, this file is located at /etc/apt/sources.list.
Below, I have included the default content for the sources.list file, as it was in version 16.04, which I found here.
Each line starting with deb points to a ppa/repository used by apt to retrieve binary packages. In the stock file, each of these lines should be followed by another line, immediately below it, that looks almost identical, but instead starts with deb-src. This is where the source code packages used by apt source are stored. Together, these lines supply the content for apt-cache. When you run sudo apt update, these are the endpoints from which the Apt cache updates. When you use apt to download content, this is where it is coming from.
#deb cdrom:[Ubuntu 16.04.2 LTS _Xenial Xerus_ - Release amd64 (20170215.2)]/ xenial main restricted # See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to # newer versions of the distribution. deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial main restricted ## Major bug fix updates produced after the final release of the ## distribution. deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted ## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu ## team. Also, please note that software in universe WILL NOT receive any ## review or updates from the Ubuntu security team. deb http://us.archive.ubuntu.com/ubuntu/ xenial universe # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial universe deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates universe # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-updates universe ## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu ## team, and may not be under a free licence. Please satisfy yourself as to ## your rights to use the software. Also, please note that software in ## multiverse WILL NOT receive any review or updates from the Ubuntu ## security team. deb http://us.archive.ubuntu.com/ubuntu/ xenial multiverse # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial multiverse deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates multiverse # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-updates multiverse ## N.B. software from this repository may not have been tested as ## extensively as that contained in the main release, although it includes ## newer versions of some applications which may provide useful features. ## Also, please note that software in backports WILL NOT receive any review ## or updates from the Ubuntu security team. deb http://us.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse # deb-src http://us.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse ## Uncomment the following two lines to add software from Canonical's ## 'partner' repository. ## This software is not part of Ubuntu, but is offered by Canonical and the ## respective vendors as a service to Ubuntu users. # deb http://archive.canonical.com/ubuntu xenial partner # deb-src http://archive.canonical.com/ubuntu xenial partner deb http://security.ubuntu.com/ubuntu xenial-security main restricted # deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted deb http://security.ubuntu.com/ubuntu xenial-security universe # deb-src http://security.ubuntu.com/ubuntu xenial-security universe deb http://security.ubuntu.com/ubuntu xenial-security multiverse # deb-src http://security.ubuntu.com/ubuntu xenial-security multiverse
I am guessing that if you were to save this to a file and diff it against your current sources.list, there would be discrepencies. From the info that you provided, it looks like your copy has a line that refers to https://download.sublimetext.com/apt/stable/Packages. The easiest way to fix your error is probably to find that line and comment it out by adding a hashtag ('#') to the beginning of the line. In case you have a reason to keep that repository active for whatever reason, you can always undo this step after the upgrade by removing the hashtag.(assuming the upgrade doesn't rewrite the file when apt is upgraded.) The same is true for any 'deb' or 'deb-src' line in the file.
After saving, you will need to run:
sudo apt-get update
to get apt to notice the change. The same will be tru for any changes you make in this file.
If this does not work, I would recommend adding a '#' to the beginning of each line that does not already have one, and then below it,copy and paste the stock content from above directly below it.
This will have the effect of reverting your ppas to the stock configuration and fixing your uprade command. Notice
