See also:

If the passenger’s frontal airbag OFF indicator illuminates and the ON indicator turns off even when the front passenger’s seat is occupied by an adult
This can be caused by the adult incorrectly sitting in the front passenger’s seat. Turn the ignition switch to the “LOCK” position. Ask the front passenger to set the seatback to the upright position ...

Forward and backward adjustment
Pull the lever upward and slide the seat to the desired position. Then release the lever and try to move the seat back and forth to make sure that it is securely locked into place. ...

Effect vehicle modifications made for persons with disabilities may have on SUBARU advanced frontal airbag system operation (U.S. only)
Changing or moving any parts of the front seats, seat belts, front bumper, front side frame, instrument panel, combination meter, steering wheel, steering column, tire, suspension or floor panel c ...