Why does an error appear when calling an async method?

Question

C++ / C#

Method taken from msdn

private async Task<FaceAttributes[]> UploadAndDetectFaces(string imageFilePath)
        {
            try
            {
                using(Stream imageFileStream = File.OpenRead(imageFilePath))
                {
                    var faces = await faceServiceClient.DetectAsync(imageFilePath);
                    var faceAttributes = faces.Select(face => face.FaceAttributes);
                    return faceAttributes.ToArray();
                }
 
            }
            catch (System.Exception)
            {
                return new FaceAttributes[0];
            }
        }

And his challenge. Taken from there

FaceAttributes[] faceAttributes = await UploadAndDetectFaces(fileNames[0]);

Answer 1

external method does not have async attribute async
void method() { await other_asyncmethod(); }