VB .NET FileUpload using ChunkUpload() error lookup_failed/not_found...

I'm trying to use this code to upload a 12mb file in [Backups] folder. I converted this code from Git dotnet C# example to VB .NET code, but I get Error [File_Not_Found], when I debug this code I noticed that sessionID never returned with value either.

Await ChunkUpload("/Backups", "C:/File.png, (Int(ChunkSize)))

Private Async Function ChunkUpload(Ipath As String, chunkSize As Integer) As Task
chunkSize = 4096 * 1024 '4mb
Using stream = New MemoryStream(File.ReadAllBytes(Ipath))
Dim numChunks = Int(Math.Ceiling(stream.Length / chunkSize))
Console.WriteLine("Chunk upload file...")
Dim buffer() As Byte = New Byte(chunkSize) {}
Dim sessionId As String = String.Empty
Dim byteRead = stream.Read(buffer, 0, chunkSize)
For idx As Integer = 0 To numChunks - 1
idx += 1
Console.WriteLine("Start uploading chunk {0}", idx)
byteRead = stream.Read(buffer, 0, chunkSize)
Using memStream = New MemoryStream(buffer, 0, byteRead)
If (idx = 0) Then
Dim result = Await DBX.Files.UploadSessionStartAsync(body:=memStream)
sessionId = result.SessionIdElse
Dim Cursor As UploadSessionCursor = _
New UploadSessionCursor(sessionId, (CLng(chunkSize * idx)))
If (idx = numChunks - 1) Then
Await DBX.Files.UploadSessionFinishAsync(Cursor, _
New CommitInfo("/Backups" + "/" + "File.png"), memStream)
Else
Await DBX.Files.UploadSessionAppendV2Async(Cursor, body:=memStream)
End If
End If
End Using
Next
End Using

While this code works fine with me with less large files 

Await DBX.Files.UploadAsync("/Backups" + "/" + "File.png",
                                                  WriteMode.Overwrite.Instance, True, Nothing, False, Nothing, False, memorystream)

Thank you.