I would suggest you let a Ford Dealer diagnose the error. Seat belt pretension er should have lit up the seat belt light not the airbag light.
There should also be a sensor in the seat that is set by weight to indicate there is a passenger in the seat. If that sensor isn't working, in an accident, the airbag won't deploy.
A dealer my charge you $50 to $100 to diagnose the issue but with the complications of sensors and wiring and computer built into vehicles these days, it's easy to miss the actual cause.
You can do the repair, but again.. Segust you have dealer do the diagnosis.
Don