There is a pending asynchronous operation, and only one asynchronous operation can be pending concurrently
Needed to add the following line of code.
Request.Content.LoadIntoBufferAsync().Wait();
This was added just before the line that was erroring out.
So it now looks like this.
var provider = new MultipartFormDataStreamProvider(root); Request.Content.LoadIntoBufferAsync().Wait(); //// Read the form data and return an async task. await Request.Content.ReadAsMultipartAsync(provider);