We are sorry for the trouble. As described yoiu are trying to comine the certified signature file with other file and getting the error 'Unsupported file'
Once the PDF file is signed it gets locked and protected for futher editing. We can not combine such files with other PDFs. However, you may try to create a portfolio of the files and see if that fulfills your requirement. For more information about creating the PDF Portfolio please refer to the steps shared in the help article https://helpx.adobe.com/in/acrobat/using/create-customize-pdf-portfolios.html