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Greetings!
I have had the same problem.
I have bought Office 365, but there is a very persistent pirated installation of a previous owner's pirated Office 2016 Pro Plus that still says "product is not activated" with that annoying pop-up window.
I have followed all the "fix it" (actually I ran the fix it module 3 times and rebooted 3 times for good measure) and then also the manual guides to the point, and the problem still persists.
I am running Win 10. Is there any way of absolutely and completely removing any trace of this Office pro plus 2016?