Essence Capture

• WWizards of the Coast2019-01-25
  Official Ruling
  You can't cast Essence Capture without a target creature spell. If either target is illegal when Essence Capture tries to resolve, the other is still affected as appropriate.
• WWizards of the Coast2019-01-25
  Official Ruling
  A creature spell that can't be countered is a legal target for Essence Capture. The spell won't be countered when Essence Capture resolves, but you'll still put a +1/+1 counter on the target creature.

Rulings via Scryfall