The plot "The Red wedding" has this card text :
"When revealed, the opponent to your left chooses 1 Lord and 1 Lady character, if able. Then, you must choose and kill 1 of those characters. The other claims 2 power."
We have an effect that looks like : "Choose X, if able. Then do Y"
The same is true for A Game of Cyvasse.
"Challenges: Each player must choose and kneel a character with an Intrigue icon he or she controls, if able. Then, the player who knelt the character with the highest STR may choose and return a character to its owner's hand."
However in our meta we play those effects differently. In the case of Game of Cyvasse if one player don't have character with INT icon we still aplly "Then Y".
However for The Red wedding we dont apply the "Then Y" part if the player cannot choose both a Lord and a Lady.
Personnaly I think the "if able" word means you will apply the "Then Y" part of the effect. Because the phrase "Do X, if able" is always true no matter if you actually did X or not, you "did it if able" successfully.
) But I did email Nate, hence the acceptance. But here's Nate's reply