Hi Leslie,
Thank you for reaching out.
As you have mentioned above, you are trying to combine the files in Google drive. However, the add files window does not provide the Google Drive option. It is a designed behavour.
The window shows the options added on your desktop. If you install the Google Drive for desktop, you will be able to see the option in the add the files window and can combine files to a single PDF.
The other options is that you can open all the files directly from Google drive folder added in the application itself. Then go to Tools > Combine PDF > Add open files. All the files will be combined to a single PDF.
Let us know if you have any questions.
Thanks,
Meenakshi