title: How do I resolve "The type or namespace 'System' could not be found" with UWP 6.2.10?
body: <p>I'm trying to build a project using Microsoft.NETCore.UniversalWindowsPlatform version 6.2.10 and I get numerous build errors such as:</p>
  
  <pre><code>error CS0246: The type or namespace 'System' could not be found (are you missing a using directive or an assembly reference?)
  error CS0400: The type or namespace name 'System' could not be found in the global namespace (are you missing an assembly reference?)
  error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib'.
  error CS0518: Predefined type 'System.Object' is not defined or imported
  error CS0518: Predefined type 'System.Boolean' is not defined or imported
  error CS0246: The type or namespace name 'AssemblyTitleAttribute' could not be found (are you missing a using directive or an assembly reference?)
  </code></pre>
  
  <p>Other developers on my team are able to build the project successfully using the exact same code.</p>
  
  <p>If I downgrade the NuGet package Microsoft.NETCore.UniversalWindowsPlatform to version 6.2.9 without making any other changes, it builds fine.</p>
  
  <p>I tested this by creating a brand new Blank App (Universal Windows) project in Visual Studio 2019, which uses version 6.2.9 of the NuGet package by default. I didn't make any changes to the template created by Visual Studio. This builds fine. Then I upgraded the NuGet package to version 6.2.10 without making any other changes. It fails with build errors similar to the errors above.</p>
  
  <p>How do I resolve these errors?</p>