Hey Jessicaw!
We're sorry for the trouble and for the delayed response. The record shows that you have an active subscription of Adobe PDFPack on your Adobe ID(email).
Try combining the PDF files through a browser like Chrome, or Firefox, or Safari using the URL Adobe Document Cloud and check.
For more information about the Adobe PDFPack, you may take help from the Adobe article Using Document Cloud Web apps and services | Adobe PDF Pack
If you are using the freeware Adobe Reader DC application and trying to combine the PDF files and now experiencing the issue, then sign out from Adobe Reader, reboot the machine.
Launch Adobe Reader back, sign-in, then check for any pending updates from the help menu of Adobe Reader>then select check for updates.
Reboot the machine after installing the update and try combining the PDF files.
If you don't have Adobe Reader desktop application installed, you may download it from the URL Adobe - Adobe Acrobat Reader DC Distribution
If you experience any issue, try connecting the machine to a different network environment like a guest wireless network or a mobile hotspot and try combining the files again.
Thanks,
Anand Sri.