How do I activate my computer?

I downloaded ADE months ago, but was never able to copy ebooks, which I had bought onto my e-reader. After a lot of troubleshooting & contacting support, I came to the conclusion that the reason for the problem is I never “activated” or “authorised” my computer.

When I try to activate/authorise my computer it gives me the following error message: “You can only authorize this computer with an ADOBE ID that hasn’t been previously used to authorize any other computer or mobile devices. Please try again.”

I have since uninstalled and re-installed ADE, I have gone through my ADOBE ID details on the Adobe.com website and reset my password.

The error message persists?

What is your advice?