when i combine file this box has show.
i can't combine any pdf file in my laptop.
this usually means that PDFCreator isn't able to find an application to print a specific type of file, in this case PDF. Often it is enough to open the Adobe Reader and to click the "make Adobe Reader my default PDF application" button.
A work around which should work in any case is to print the files you'd like to combine to the virtual PDFCreator instead of using the right click-> convert with PDFCreator method.
Thank you. Now it's ok.