If you have a UFO plane , then you not have the right plane(needed plane). On the right side here you have a link the the right plane. jetabouts 737-800w. If you already have exact that plane subscribed , then unsubscribe it please(with SA closed) and subscribe it again, , then go to mod status in game and click download missing mods , then it should work. (that is a loading bug of steam)

i get additionally to the 73W an ufo aircraft. which one is that supposed to be?