var filename = Path.Combine(_env.WebRootPath, "files", "data.xlsx");
            using (SpreadsheetDocument myDoc = SpreadsheetDocument.Create(filename, SpreadsheetDocumentType.Workbook))
            {
                WorkbookPart workbookpart = myDoc.AddWorkbookPart();
                workbookpart.Workbook = new Workbook();
                // Add a WorksheetPart to the WorkbookPart.
                WorksheetPart worksheetPart = workbookpart.AddNewPart<WorksheetPart>();
                SheetData sheetData = new SheetData();

                //add the first row
                Row firstRow = new Row();
                firstRow.RowIndex = (UInt32)1;

                //create a cell in A1 (the upper left most cell of the merged cells)
                Cell dataCell = new Cell();
                dataCell.CellReference = "A1";
                CellValue cellValue = new CellValue();
                cellValue.Text = "Raw Matrials and Price List";
                dataCell.Append(cellValue);
                firstRow.AppendChild(dataCell);
                sheetData.AppendChild(firstRow);

                //add the second row
                Row secondrow = new Row();
                secondrow.RowIndex = (UInt32)2; 
                //create a cell in A2 (the upper left most cell of the merged cells)
                Cell secdataCell = new Cell();
                secdataCell.CellReference = "A2";
                CellValue seccellValue = new CellValue();
                seccellValue.Text = "From: 121";

                Cell secdataCell2 = new Cell();
                secdataCell2.CellReference = "C2";
                CellValue seccellValue2 = new CellValue();
                seccellValue2.Text = "To: 250";

                secdataCell.Append(seccellValue);
                secdataCell2.Append(seccellValue2);
                secondrow.AppendChild(secdataCell);
                secondrow.AppendChild(secdataCell2);
                sheetData.AppendChild(secondrow);

                //based on the table data to add row and cell.

                // Add a WorkbookPart to the document.
                worksheetPart.Worksheet = new Worksheet(sheetData);

                //create a MergeCells class to hold each MergeCell
                MergeCells mergeCells = new MergeCells();

                //append a MergeCell to the mergeCells for each set of merged cells
                mergeCells.Append(new MergeCell() { Reference = new StringValue("A1:d1") });
                mergeCells.Append(new MergeCell() { Reference = new StringValue("A2:b2") });
                mergeCells.Append(new MergeCell() { Reference = new StringValue("c2:d2") });

                worksheetPart.Worksheet.InsertAfter(mergeCells, worksheetPart.Worksheet.Elements<SheetData>().First());

                //this is the part that was missing from your code
                Sheets sheets = myDoc.WorkbookPart.Workbook.AppendChild(new Sheets());
                sheets.AppendChild(new Sheet()
                {
                    Id = myDoc.WorkbookPart.GetIdOfPart(myDoc.WorkbookPart.WorksheetParts.First()),
                    SheetId = 1,
                    Name = "Sheet1"
                });
            }