If the "Confirm" button is grayed-out, it's because there is no license plate registered on your account, which may be mandatory for making reservations. 

A notification next to the button "confirm" inform you if you need to register a car plate : 

To register your car plate on Sharvy, you can go to : "Account > My account" and add a véhicle : 

Once registered, you should be able to confirm your request.