Runes can target and be attached to any permanent, even one that isn't currently a creature or an Equipment. You can cast a Rune choosing a Vehicle as the target, for example. The Rune's enters-the-battlefield ability will trigger, and you'll draw a card. The Rune won't do anything while the Vehicle isn't a creature, but if it becomes one later, the appropriate ability will start applying.

If the target of an Aura spell is an illegal target as that spell tries to resolve, it won't resolve, it won't enter the battlefield, and none of its enters-the-battlefield triggered abilities will trigger.