Of course you need luck on penalties. You need luck in football match as well. Croatia played great, but we had luck as well. Kane had a clear chance for 2:0, but frame of the goal saved us. So that is luck.
But as in football match, you need skill and cool head for penalties. Or is anyone here claiming that most successful football nations are just plain lucky? Because Germany, Brazil, Argentina, Italy... Well, they win most of their penalty shootouts. And by some chance, those are the countries that won WC most times.
Player needs to be comfortable with penalties. Messi is ok penalty taker, but just ok, not even good. And no one can deny that he is really skilled football player. Modric is bad at penalties, but also, no one can deny his skill.
On the other hand I remember Robert Prosinecki really well. That guy took most of penalties for his teams. He missed just 2, and both misses came in the same game (Croatian league, vs Hajduk). And by some chance, goalkeeper was his team mate in national squad. And goalkeeper used to stay longer in practice to take penalties with Prosinecki. So skill mostly.
If Croatia vs France match goes to penalties, I really wish Modric doesn't take penalty this time. Because he really isn't good at them.