You can do it in the lobby... but not with the reception - you will still need to do it via Marriott website or via the phone, you can't do it at the desk. The reason is the people at reception will have no real access to your marriott bonvoy account (for rather obvious reasons like fraud and identity protection I speculate), so they wouldn't be able to verify your identify and issue a certificate.

Be aware: The points (and most hotel) reservations are locked/expired at midnight local time, sometimes before, at least online. I've never tried to book a room the prior night after midnight, never mind with points... so if you're arriving late at night you may be stuck with nothing but a paid room option locally.