C++/CLI can not instantiate classes like you can do it in C#, multiple in one time. And sometimes you must use AppendChild and not just Append method but I don't know why
// *** same code like you
// Add a WorksheetPart to the WorkbookPart.
WorksheetPart^ worksheetPart = workbookpart->AddNewPart<WorksheetPart^>();
SheetData^ sheetData = gcnew SheetData();
worksheetPart->Worksheet = gcnew Worksheet();
worksheetPart->Worksheet->AppendChild(sheetData);
// same code like you
sheets->AppendChild(sheet); // *->AppendChild not just *->Append
workbookpart->Workbook->Save();
// Close the document.
spreadsheetDocument->Close();