writing-testable-code | Sample code for my course on writing testable code | Learning library

So - I'm pretty much a noob at C#, I've gone through a lot of tutorials and browsed through some large C# projects from work and built them, and done some other minor things. I'm going through a course on writing testable code on Pluralsight. He has a public Github repo for the code examples, writing-testable-code. I connected to the repo and downloaded it okay into a local Git repo. I was able to download all the packages from NuGet and they are all showing as the version he used (a few have updates, but I figured updating might break things).

I can't figure out how to run this code, build it, or run the tests in it.

My issue is - I open the solution, and there are a bunch of files and folders - each module/chapter is split into folders (i.e. Module1/Easy, Module1/Hard, Module2/Easy, etc.). I want to build the Module1/Easy folder, including unit test examples, and run the tests.

When reviewing Module1/Easy, it has 3 files that should build okay - the program.cs has a main() and looks like a console app, the Calculator.cs has a simple class, and the CalculatorTests.cs has unit tests built for Nunit. The solution has NUnit, Castle.core, and things from later modules (Moq, AutoMoq, Unity, Ninject, etc.). It didn't seem to have a VS runner, so I added Nunit3TestAdapter - the guy in the course has resharper installed, which I don't, and he was using the Resharper test runner, which would explain why he didn't include it.

I tried setting the "Module1/Easy/Project.cs" file the "Set as Startup Item", since it has a main and looks setup as a console app. However, running it (the "Start" button turned into a "Program.cs" button), it fails saying it can't run a dll. The tests aren't showing up in the Test Explorer like some other small projects I've built from examples.

I'm not sure where to go from here. On the Build menu is only a "Build Solution" and one about Code Analysis - I'm used to a lot more options here. It feels like I have to turn this folder into a project, maybe? I can always reinstall the packages - but what is the best solution here?

I've run into this before on other book, tutorial, or class repos, but finally decided to figure out how to get this one working. I appreciate any help!

1. I'm running Visual Studio Community 2017 at the moment.
2. I can post some of the files, but the repo is publically available, and not sure exactly what to post to help.

1. Per Biker-Dude's answer, I switched the project to build a console app rather than a dll, and now I get a compile-time error for having multiple entry points (i.e. every module and sub folder has its own Main() function and should probably be a separate project).
2. After #1, I removed all folders but one from the solution, it will then compile, run the tests, etc. - but I eventually want to be able to at least separately compile every sub-folder - what's the best way?