We have checked your account and found that you are a subscriber of Adobe PDF Pack. Adobe PDF Pack is a bundle of Adobe Document Cloud services. With it, you can easily create, combine, organize, sign, and send PDF files online. You can also convert PDF files into editable Word or Excel files.
You can access the services online and through the desktop application Adobe Reader DC. But "Combine PDF" feature only works through the web browser.
Go to the website Adobe Document Cloud and you may Sign Out and Sign In once, and see if that helps.
I have accessed the website, with one of your phone representatives, but the combine button is still gray.
Should I cancel may subscription?
This shouldn't be happening if you are a PDF Pack subscriber. There might be some issue with the email you are signing in or the web browser you are working on.
Since you have canceled the subscription, so there is nothing that we can suggest you further. Let us know if there is any further assistance is required in future.