Can't activate office professional plus 2016

I am a college student.  We buy Office Professional Plus 2016 from them and download it.  It comes with a product key.  When I go to my office and enter the correct numbers/letters, it tells me to go to another link for office 365.  There told me to go to the web office.com/setup.  It tells me the same thing.  Why can I not activate my office?  Where do I go?  I have been told three places that say go somewhere else.