What happens when you can't pay for Deflect on a mandatory triggered ability (like Leona's stun when attacking)?

Ruling: For triggered abilities targeting units with Deflect, you may choose not to pay the Deflect cost. If you decline to pay (or cannot pay), the triggered ability does not finalize on the chain and its effect does not happen. You can still perform the action that triggered it (e.g., attacking with Leona).

Sequence:

• The triggering action (e.g., Leona attacks) is legal and proceeds
• The triggered ability enters the chain as pending
• When Deflect cost must be paid, you may choose to decline
• If declined, the ability is removed from the chain and never finalizes
• The ability's effect does not occur

Nuances:

• This only applies to triggered abilities - spells and activated abilities targeting units with Deflect have a MANDATORY additional cost that must be paid
• For spells/activated abilities, if you cannot pay Deflect, the action is reversed and the spell stays where it was (doesn't go to trash)
• The choice to decline paying Deflect for triggered abilities is optional even if you have the resources to pay
• Costs for spells are locked in when played, even if game state changes due to reactions afterward
• However, Deflect costs for reflexive triggers generated by spells are evaluated at the time those triggers are generated