Facebook advideos upload, issue while uploading the video_file_chunk

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C#

Facebook-API

Hello All,
I'm trying to upload an Ad Video to facebook using the steps given in below url and Facebook SDK for Dot.NET.

https://developers.facebook.com/docs/marketing-api/advideo/v3.2[^]

My Code:

var fbClient = GetFB();
                FileStream fs = new FileStream("E:\\cnp.mp4",
                                   FileMode.Open,
                                   FileAccess.Read);
                BinaryReader br = new BinaryReader(fs);

                dynamic parameters = new System.Dynamic.ExpandoObject();
                parameters.upload_phase = "start";
                Console.WriteLine("file_size: {0}", fs.Length);
                parameters.file_size = fs.Length;
                dynamic result = fbClient.Post("/" + FBAccountID + "/advideos", parameters);
                Console.WriteLine("upload_session_id: {0}", (string)result["upload_session_id"]);
                string upload_session_id = (string)result["upload_session_id"];
                Console.WriteLine("Video Id: {0}", (string)result["video_id"]);
                Console.WriteLine("start_offset: {0}", (string)result["start_offset"]);
                Console.WriteLine("end_offset: {0}", (string)result["end_offset"]);

                int startOffset = 0;
                int endOffset = int.Parse((string)result["end_offset"]);

                while (endOffset > startOffset)
                {
                    byte[] buff = new byte[endOffset - startOffset];
                    buff = br.ReadBytes((endOffset - startOffset));

                    dynamic transfer = new System.Dynamic.ExpandoObject();
                    transfer.upload_phase = "transfer";
                    transfer.upload_session_id = upload_session_id;
                    transfer.start_offset = startOffset;
                    transfer.video_file_chunk = buff;
                    //--Getting Error

                    dynamic TransferResult = fbClient.Post("/" + FBAccountID + "/advideos", transfer);
                    Console.WriteLine("start_offset: {0}", (string)TransferResult["start_offset"]);
                    Console.WriteLine("end_offset: {0}", (string)TransferResult["end_offset"]);
                    startOffset = int.Parse((string)TransferResult["end_offset"]);
                    endOffset = int.Parse((string)TransferResult["end_offset"]);
                }

                dynamic finish = new System.Dynamic.ExpandoObject();
                finish.upload_phase = "finish";
                finish.upload_session_id = upload_session_id;
                finish.title = "Video title";
                dynamic result2 = fbClient.Post("/" + FBAccountID + "/advideos", finish);
                Console.WriteLine("success: {0}", (string)result2["success"]);
                if ((string)result2["success"] == "true")
                {

                }

The First request for Start was successful and we are getting the VideoID and Upload SessionID, and the second request is failing with error

Facebook.FacebookOAuthException: '(OAuthException - #2) Service temporarily unavailable'

Error User Title: "Video Upload Time Out"

Error User Message: "Your video upload timed out before it could be completed. This is probably because of a slow network connection or because the video you're trying to upload is too large. Please try again."

Facebook.FacebookOAuthException occurred
HResult=0x80131500
Message=(OAuthException - #2) Service temporarily unavailable
Source=Facebook
StackTrace:
at Facebook.FacebookClient.ProcessResponse(HttpHelper httpHelper, String responseString, Type resultType, Boolean containsEtag, IList`1 batchEtags)
at Facebook.FacebookClient.Api(HttpMethod httpMethod, String path, Object parameters, Type resultType)
at Facebook.FacebookClient.Post(String path, Object parameters)
at WebApplication1._Default.Facebook() in C:\Users\suneel.CLICKANDPLEDGE\documents\visual studio 2017\Projects\TestConsoleApp\WebApplication1\Default.aspx.cs:line 132
at WebApplication1._Default.Page_Load(Object sender, EventArgs e) in C:\Users\suneel.CLICKANDPLEDGE\documents\visual studio 2017\Projects\TestConsoleApp\WebApplication1\Default.aspx.cs:line 16

What I have tried:

I'm passing the byte[] to video_file_chunk. As per the documentation it says binary file.

I tried manipulating

video_file_chunk

but getting the same error,

Thanks in advance for any help.