Unable to add project reference in Visual Studio 2015

You should add project to your solution first; after adding project to your solution, you can reference it.

To add an existing project to your solution:

Add reference menu:

Add references dialog:

Right click on your solution file in Solution Explorer window, click Add and select Existing Project...

Once the project is a part of the solution, you will be able to add project reference.

For me issue was due to difference in Target framework. One was .NET Framework 4.5 and other was .NET Framework 4.5.2

Verify if the Target framework is same for your project and the project reference that you are trying add reference.

(Right click on project -> properties -> Application Tab(Target frame work))

In case anyone else runs into this here is something that I found. I had almost the exact same issue in VS 2015. I created a console app and then a class library project but could not add a reference to the class library.

It turns out I didn't create a class library project as I had thought, but rather a class library (package) project. These are different things and I was unaware of what the (package) part meant when I selected it. This is for creating Nuget packages, apparently.

I'm not sure if this is something new to VS 2015 but I had not run across it before and it caused me some grief. The error message ('A reference to ClassLibrary1 could not be added') was totally worthless in trying to debug it.