This is really unfortunate. But the terms on this are very clear - I even got several (at least 2) emails reminding me of selecting the benefit should I not want the automatic assignment of SNA (and these email also stated that the selection cannot be undone after the deadline).
Probably the only thing you can do is try and ask for an exception. I would be kind though as the only thing which someone did wrongly is actually trying to help you and put a cert into your account (which then apparently got pulled by audit).