Luke’s comment, that the apostles “understood none of these things; this saying was hid from them, and they did not grasp what was said,” is puzzling.
Jesus’ statement seems perfectly clear, entirely straightforward. There’s nothing in it that’s difficult to grasp, conceptually. It’s not Hegel or Kant.
So what does it mean that they didn’t understand or grasp what Jesus was saying?
I suppose that one possibility is simply this: He was so dominant, his personality so alive, his leadership so absolute, and his power so manifest, that they just couldn’t comprehend that he would die, let alone that he would be seized by others and forcefully put to death. They were probably still thinking of him as the Messiah in the (more or less) standard Jewish way: He would, pretty soon, triumph over his enemies, drive the Romans out, and establish the earthly Kingdom of Heaven. They didn’t fully grasp, maybe didn’t really grasp at all, the fact that his Messiahship would be enacted through suffering and death.