Problem to Append line

Error	2	'System.Collections.Generic.IEnumerable<DocumentFormat.OpenXml.Spreadsheet.Sheet>' does not contain a definition for 'Append' and no extension method 'Append' accepting a first argument of type 'System.Collections.Generic.IEnumerable<DocumentFormat.OpenXml.Spreadsheet.Sheet>' could be found (are you missing a using directive or an assembly reference?)	C:\App0\App5\Program.cs	166	40	App5

                            IEnumerable<Sheet> sheets = spreadSheet.WorkbookPart.Workbook.Descendants<Sheet>().Where(s => s.Name == vSN);
                            if (sheets.Count() == 0)
                            {
                                // Add a WorkbookPart to the document.
                                WorkbookPart workbookpart = spreadSheet.AddWorkbookPart();
                                workbookpart.Workbook = new DocumentFormat.OpenXml.Spreadsheet.Workbook();

                                // Add a WorksheetPart to the WorkbookPart.
                                WorksheetPart worksheetPart = workbookpart.AddNewPart<WorksheetPart>();
                                worksheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(new SheetData());

                                Sheet sheet = new Sheet()
                                {
                                    Id = spreadSheet.WorkbookPart.
                                        GetIdOfPart(worksheetPart),
                                    SheetId = 1,
                                    Name = vSN
                                };
                                sheets.Append(sheet);

                            }
                            

Many Thanks & Best Regards, Hua Min