How can I use Luka´s teleportation ability, he is not in my party nor in the reserve?
Do I have to get Luka in my party and how can I do that?

What part of the story are you at?

I am in the museum standing in front of the shelf and looking for the keycards. I am playing Kasane and can't find the keycards.
Perhaps there is a difference if you play Kasane or Yuito?

Worry not. Just continue the main story.

you'll get him later. he's basically a Ying to Shiden's Yang, a walking lockpick.