If Setessan Champion leaves the battlefield after its ability has triggered, you still draw a card even though you won't put a +1/+1 counter on Setessan Champion.

A constellation ability triggers whenever an enchantment enters the battlefield under your control for any reason. Enchantments with other card types, such as enchantment creatures, will also cause constellation abilities to trigger.

An Aura spell that has an illegal target when it tries to resolve doesn't resolve and is instead put into its owner's graveyard. It doesn't enter the battlefield, so constellation abilities don't trigger.