I have never been able to get checked in online with Saudia going to/from the US. As long as you have the ticket confirmation number, you should be fine, and they will likely just ask you to come to a Saudia check-in desk at the airport to take your ID info and make sure you have the proper visa paperwork/eligibility. If you booked an itinerary that includes carriers besides Saudia, they will also probably make you check in again at the flight gate to get new boarding passes printed directly from Saudia.