This registration is mandatory to comply with Italian public security regulations — specifically Art. 109 of T.U.L.P.S. (Consolidated Text of Public Security Laws) and the Ministerial Decree of January 7, 2013 — which require the registration and reporting of all accommodated guests to the authorities.
Registration must be completed before arrival at the property: as it is a self-check-in, there will be no staff present to register you in person.
Furthermore, by completing the registration in advance, you will receive all instructions via email, which you can also print to have with you in case your phone battery is dead upon arrival.
Without this information, it will not be possible to comply with Italian law or to send you the access codes through the online check-in procedure.