Draining Whelk

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2021-03-19
  Official Ruling
  If the target spell is an illegal target by the time Draining Whelk's ability tries to resolve, the spell doesn't resolve. You don't put any counters on Draining Whelk. If the target is legal but the spell can't be countered, you do put counters on Draining Whelk.
• WWizards of the Coast2021-03-19
  Official Ruling
  For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
• WWizards of the Coast2021-03-19
  Official Ruling
  Draining Whelk's ability counters the target spell even if you can't put counters on Draining Whelk, most likely because Draining Whelk left the battlefield before the ability resolved.

Rulings via Scryfall