How do I get the error message from an HttpResponse object in WebAPI?

As you figured in your comment, you could either use response.Content.ReadAsAsync<HttpError>() or you could also use response.TryGetContentValue<HttpError>(). In both these cases, the content is checked to see if its of type ObjectContent and the value is retrieved from it.

You can try the following:

var errorContent = await response.Content.ReadAsAsync<HttpError>();
Assert.That(errorContent.Message,Is.EqualTo("No Permission"));

Try this one. response.Content.ReadAsAsync<HttpError>().Result.Message;