Build error: "Metadata file '.dll' could not be found" in WPF project

Hello everyone! I'm a beginner in C# and WPF (learning from an old textbook for Visual Studio 2008, C# 3.0). I have a solution with two projects: a main WPF project (MyApp) and a class library (MyLibrary). In MyApp, I added a reference to MyLibrary, but when I try to build the solution, I get the error: "Metadata file 'C:\Projects\MyApp\MyLibrary\bin\Debug\MyLibrary.dll' could not be found" Here is the code from MyLibrary (a simple class): ```csharp using System; namespace MyLibrary { public class Calculator { public int Add(int a, int b) { return a + b; } } } ``` In MyApp, I'm trying to use this class: ```csharp using System; using System.Windows; using MyLibrary; namespace MyApp { public partial class MainWindow : Window