Could not load file or assembly 'Microsoft.VisualStudio.TestTools.UITest.WindowsStoreUtility, Version=15.0.0.0

I'm trying to write an object framework for an application I will be testing. It's in very simple form at the moment. Written in C# on Visual Studio 2017. When I try to include references to Windows classes, e.g. 'WinWindow', I get run time errors as such:

Exception thrown: 'System.IO.FileNotFoundException' in Microsoft.VisualStudio.TestTools.UITesting.dll

An unhandled exception of type 'System.IO.FileNotFoundException' occurred in Microsoft.VisualStudio.TestTools.UITesting.dll

Could not load file or assembly 'Microsoft.VisualStudio.TestTools.UITest.WindowsStoreUtility, Version=15.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.

I have tried to find this library, but it doesn't exist on my computer and I can't find any reference on line. What do I do now?