We apologize for the inconvenience caused, as per the description above, you are not able to combine documents, Is that correct?
The record shows that you are subscribed to Adobe PDFPack services on your current Adobe ID, please try combining the PDF files using the URL https://cloud.acrobat.com/convertpdf with your Adobe ID and password on a different browser like Chrome, or Firefox, or Safari and see if this brings any difference.
If you are using the free desktop version of Adobe Reader and get the error message, please sign out and sign back in with your current Adobe ID and password in Adobe Reader and check for any pending updates and install them, and reboot the machine and try combining the files.
For more troubleshooting, you may refer to Troubleshooting Create PDF, Export PDF, Adobe Send, Fill & Sign … services(PDFPack or Document Cloud...
Feel free to update this discussion for any further assistance.