Eutropia the Twice-Favored

• WWizards of the Coast2020-01-24
  Official Ruling
  If an enchantment creature enters the battlefield under your control, Eutropia’s ability can target that creature.
• WWizards of the Coast2020-01-24
  Official Ruling
  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.
• WWizards of the Coast2020-01-24
  Official Ruling
  Eutropia can be the target of its own triggered ability.
• WWizards of the Coast2020-01-24
  Official Ruling
  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.
• WWizards of the Coast2020-01-24
  Official Ruling
  Eutropia’s ability can target a creature that already has flying. It still gets a +1/+1 counter. Multiple instances of flying on the same creature are redundant.

Rulings via Scryfall