I had a similar thing with my 147. I used a local garage for its last service - they are Bosch agents & have a very sophisticated Bosch diagnostic set-up, despite which, at the time of the service they couldn't reset the service indicator. They phoned me a few months later & said that they had a software upgrade so they would try again. Even then, all they were able to do was set the mileage to zero, but without the warnings which should appear when a service is due.
For reasons known only to them, Alfa has set the system up so that only garages which have their equipment can rest service indicators. . . other manufacturers are happy to do it with just a few presses of buttons on the dash.
Where are we going & why am I in this handcart?