Can I play an Action spell like Rebuke in response to an opponent playing a unit with an ability (like Pit Rookie) on their turn?

Ruling: No, you cannot play an Action card on your opponent's turn in response to a unit ability. You can only play Action cards on your opponent's turn when you have focus during a showdown and the chain is empty.

Nuances:

• The unit ability does create a chain
• Only Reaction cards can be added to chains
• You may react with Reaction cards, but not Action cards during a chain