Lupa Shadow Swipe

Why can't my Lupa learn Shadow Swipe? I got it out of an Intermediate Chest, so it seems that Lupa should be able to learn it, but it doesn't show up when I look at my available skills. I understand that it's not the greatest skill in the world at 50 power, but why can't Lupa learn it?

lupa is psychic type
shadow swipe is ghost type
it doesnt make sense if psychic can learn ghost type moves uh