How do I add a reference to UnityEngine.Purchasing to fix this error?

Unity Engine
1

FileNotFoundException: Could not load file or assembly ‘UnityEngine.Purchasing, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null’ or one of its dependencies.
System.MonoType.GetFields (BindingFlags bindingAttr) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/MonoType.cs:164)
UnityEditor.MonoScriptImporterInspector.ShowFieldInfo (System.Type type, UnityEditor.MonoImporter importer, System.Collections.Generic.List1 names, System.Collections.Generic.List1 objects, System.Boolean& didModify) (at C:/buildslave/unity/build/Editor/Mono/Inspector/MonoScriptInspector.cs:74)
UnityEditor.MonoScriptImporterInspector.OnInspectorGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/MonoScriptInspector.cs:125)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[ ] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1227)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Just as the subject says. How do I fix this? (It FEELS like a path issue).

2

I ended up adding it as an existing asset to my project. That has stopped the error messages above. We’ll see if it holds.