System.Configuration.ConfigurationErrorsException was caught during uploading file to dropbox using

This Code is working fine with console application, but I am getting error with windows service saying Configuration system failed to initialize(System.Configuration.ConfigurationErrorsException was caught)

private static async Task Backup()
    {        using (var dbx = new DropboxClient("<REDACTED>")) 
        {
            string folder = "DropboxUpload";
            string file = "setup.exe";
            string content = @"D:\Share\setup.exe";            log.Info("Chunk upload file...");
            // Chunk size is 100 mb.
            const int chunkSize = 2 * 1024 * 1024;            using (var stream = new FileStream(content, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
            {
                int numChunks = (int)Math.Ceiling((double)stream.Length / chunkSize);
                byte[] buffer = new byte[chunkSize];
                string sessionId = null;
                for (var idx = 0; idx < numChunks; idx++)
                {                    log.Info("Start uploading chunk {0}" +idx);
                    var byteRead = stream.Read(buffer, 0, chunkSize);                    using (MemoryStream memStream = new MemoryStream(buffer, 0, byteRead))
                    {
                        if (stream.Length <= chunkSize)
                        {  
                          // here i am getting exception                         await dbx.Files.UploadAsync(new CommitInfo("/" +folder + "/" + file), body: memStream);
                        }
                        else if (idx == 0)
                        {
                            var result = await dbx.Files.UploadSessionStartAsync(body: memStream);                            sessionId = result.SessionId;
                        }
                        else
                        {
                            UploadSessionCursor cursor = new UploadSessionCursor(sessionId, (ulong)(chunkSize * idx));
                            if (idx == numChunks - 1)
                            {                                await dbx.Files.UploadSessionFinishAsync(cursor, new CommitInfo("/" + folder + "/" + file), memStream);                                log.Info("BackUp Completed...!!!");
                            }
                            else
                            {                                await dbx.Files.UploadSessionAppendV2Async(cursor, body: memStream);
                            }
                        }
                    }
                }
            }
        }
    }