Problem with adding a class library to my solution

Question

I have a regular Windows Form app in C#, built about 2 years ago, which includes a couple class libraries, and everything works. However, today I tried to add another class library, and this is how I did it:

1. Right-click the Solution name
2. Choose "Add"
3. Choose "New Project"
4. Select class library template

I built a class in the new project, but when I compile the solution, for 2 seconds it says 0 errors, then it switches and says 2 errors: CS0246 Type or namespace ScrubMACC could not be found (are you missing a using directive or assembly reference?) So, I checked project dependencies, and reference manager. It all seems in order, but I feel like I'm forgetting something simple! Any thoughts?