Error in generating excel

Question

I have a problem in C# when i try to generate new excel file with 4 values. When i try to open file i receive an error telling me : excel cannot open the file because the file format or file extension is not valid

When i try to debug it i have noticed that output is trowing : System invalid operation exception and i don't know how to fix this. i just want to generate excel file that has some values inside (later i will fill insert my values). For now i am just trying to create excel file that isn't corrupted.

    [HttpPost]
    public HttpResponseMessage PrintDevices()
    {

        HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
        using (MemoryStream output = new MemoryStream())
        {


            using (SpreadsheetDocument document = SpreadsheetDocument.Create(output, SpreadsheetDocumentType.Workbook))
            {
                WorkbookPart workbookPart = document.AddWorkbookPart();
                workbookPart.Workbook = new Workbook();

                WorksheetPart worksheetPart = workbookPart.AddNewPart<WorksheetPart>();
                worksheetPart.Worksheet = new Worksheet();

                Sheets sheets = workbookPart.Workbook.AppendChild(new Sheets());

                Sheet sheet = new Sheet()
                {
                    Id = workbookPart.GetIdOfPart(worksheetPart),
                    SheetId = 1,
                    Name = "Testing"
                };

                sheets.Append(sheet);

                workbookPart.Workbook.Save();


                SheetData sheetData = worksheetPart.Worksheet.AppendChild(new SheetData());

                // Constructing header
                Row row = new Row();

                row.Append(
                    ConstructCell("Id", CellValues.String),
                    ConstructCell("Name", CellValues.String),
                    ConstructCell("Birth Date", CellValues.String),
                    ConstructCell("Salary", CellValues.String));

                // Insert the header row to the Sheet Data
                sheetData.AppendChild(row);

                // Inserting each employee


                worksheetPart.Worksheet.Save();
            }
        }

         Cell ConstructCell(string value, CellValues dataType)
        {
            return new Cell()
            {
                CellValue = new CellValue(value),
                DataType = new EnumValue<CellValues>(dataType)
            };
        }

        return response;
    }