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The OSIsoft.AF.PI namespace was introduced with AF 2012 (AF SDK 18.104.22.16839). Here's the link to What's New with PI AF 2012, where the opening paragraph states you need the .NET 4.0 version of the AF SDK. Be sure your app is targeting .NET Framework 4.0. If you use LiveLibrary to look at help on the PIServer Class, you will see this note:
Which means your app should be sure to reference the 4.0 version of AF SDK, and not the 3.5 (or 2.0) version.
In Windows File Explorer:
In Visual Studio: