PDF Pack: just need to combine some files; "combine" button is grayed out and unresponsive

I purchased PDF Pack solely to be able to combine PDF files. I understand that with my plan, this can only be done on the browser and not in the desktop app. However, I am being prevented from combining files at this link. I have tried the link on multiple browsers. I have signed out of my account, cleared my cache and cookies, and signed back in. I have tried combining a wide variety of documents, not just the ones I need to combine for my job. My OS is up-to-date. I tried everything the forums suggested, to no avail. I tried everything that the tech support chat person suggested, again to no avail, and was told to go to the forums.

I just need to get the "combine" button to be blue and not gray.