Personally I would have accepted it (as per my answer back when I was a mod above), but I can't be certain if my reasoning applies 100% to Brazilian Portuguese (I'm from Portugal).
I have to say that here in this sentence "certificate" doesn't make much sense in context, but I know how frustrating it can be to get one's answer wrong on those grounds and how disheartening it may be for your progress. Hopefully you can move past it and continue with your studies!
Good luck with your studies :)
P.S. As a tangent for those of you learning European Portuguese, a college/university degree is usually referred to as "[o] curso", with [o] diploma being somewhat more formal (a synonym, but much less used). Also, "diploma" can mean any piece of papel associated with a certain achievement, not just academic.