You are missing the obvious reference to System in your usings.
using System;
Why do you need this reference and why it complains it is missing a seemingly unused method?
With await, it actually calls WindowsRuntimeSystemExtensions.GetAwaiter, an extension method over IAsyncOperation (to get the TaskAwaiter to await). Since WindowsRuntimeSystemExtensions resides in the System namespace, you need that using to get the extension method.
