Handling your sources

Your sources in Linux are the repositories you have access to that can provide your apt tool access to APT packages.
For those migrating from Windows to Linux, this can be new and confusing information as MS Windows does not have sources.
Linux is, by nature, more secure. Sources on Linux ensure more vetted packages, allowing greater security since these must be Signed and run through lintian prior to being accepted into a vetted repository (Like Launchpad.)

Sometimes, sources can be a source of frustration for a user trying to install a package.
Most distros are based off of a Base Distribution. In the case of Zorin OS, it is currently based off of Ubuntu. This is also true of Linux Mint and many others. MXLinux is based off of Antix and Debian. All of these, therefor tend to have access to the main distro repositories.
If using a distro other than Zorin OS and if you are reading this; you will need to check which main server you use in your distro. For Zorin OS, it is the Ubuntu Main Universe Repository and these follow the release code names.

Zorin OS 12 is based off Ubuntu 16.04 and the Repository code name is Xenial
Zorin OS 15 is based off Ubuntu 18.04 and the Repository code name is Bionic
Zorin OS 16 is based off Ubuntu 20.04 and the Repository code name is Focal

In following Online Guides or other sources, many of these guides fail to cover this essential timeline: The source you use must be the source for your release codename. If you are on Zorin OS 15, then this is "Bionic" and if you are following an older guide that instructs to add the "Xenial" reposiroty, this will fail. Your addition of that repository may say

Repository does not have a release file

When this happens, you know that you have the Wrong Codename. It may also show the codename, such as "Xenial" in your sources, when you know you are on Zorin 15 and it should be "Bionic" or on Zorin 16 and it should say "Focal."
When you see this, remove the repository.

Open Software & Updates and navigate to the Other Software tab. Scan the list for that repository and click on it to select it. Then click the remove button on the bottom toolbar.

IF you see this message, but you were certain that it was the right codename, it means that the maintainer of that repository has not released a package for that codename. For example, if they maintainer has only uploaded packages as high as "Bionic" but has not uploaded any for "Focal."

Again, you must remove this broken repository from your sources.

Often times, you can try navigating to their Launchpad page and selecting Package Details. Here, you can check what is uploaded where - or just grab the latest .deb package on the left hand side by clicking the > icon next to the package name. Even if they have not uploaded the package to "Focal", perhaps the "Bionic" package will still install and work, so try checking the package and its dependencies. Grab the .deb package and then open it with the tool Gdebi.

When following an online guide for installation, ensure that you maintain the Codename for the repository you are using for Zorin OS (Or your distribution) and be sure to change the offered codename in the guide for the one of the build you are using.

You may also see:

Updating from such a repository can't be done securely, and is therefore disabled by default

There can be several causes for this - the most common is that the repository is unsigned or you do not have the keys added on your system. Recall from above, Linux is more secure.
It is not advisable to install from an unsigned repository by using the -f force parameter or --allow-unauthenticated --allow-insecure-repositories. For obvious reasons. The best course of action is to remove the repo with the steps outlined above and seek out a more reliable source.
However, if you only need the keys to a repository, these can be added using the apt-key add command. This can happen when using a third-party repository for another distribution. These tend to be a case-by-case basis, so please start a thread in the General Help Forum for assistance in setting up the repository.