Thanks for considering Aspose.
Well, Actually Aspose.Cells does not create pivot table report rather sets the pivot data formats and forces MS Excel to create pivot table report and other calculation tasks based on the data source when the workbook opens into MS Excel. So you should use SaveType.OpenInBrowser rather than using SaveType.OpenInExcel. One of the many reason is when you use OpenInExcel option while saving the output generated file into MS Excel at runtime using "Open" button of the download dialog box, MS Excel could not parse the Workbook data to generate pivot table report. This is caused by the filename problem, It is the routine of IE as it appends something like "" to make it as "fileName"+ ""+ ".xls" to the original name and thus nothing to do with Aspose.Cells. (i.e.... it always adds "" to make "fileName"+ ""+ ".xls" and not like fileName.xls). In short, if a file contains pivot table, it cannot be opened using OpenInExcel SaveType option and this will apply for both i.e., if you create the file from scrach or using any template file for source data to create pivot table report. So, you should use OpenInBrowser SaveType option if the file has pivot table data in it to create pivot table report.
Kindly change your code and update to SaveType.OpenInBrowser if you are using Workbook.Save() method
//Or update to make "inline" if you are using "attachment" option in your code. i.e.,
//Response.AddHeader( "content-disposition","inline; filename=MyBook.xls");
Hope, you will get some insight. Kindly check now if it is fine. If you still find some problems or pivot table does not create fine, feel free to let us know.
If you still could not resolve it, please create a sample test and post your template file with code.