We see Ransom stabbing Marta in the chest, but there was some weird noise, and apparently the kill didn’t work. What did happen? Did the knife retract? I have no clue.
If you recall from earlier in the film, Harlan criticizes Ransom's protected lifestyle saying he is unable to tell the difference between a stage prop and a real knife. That was a bit of foreshadowing, as Ransom grabs a knife to kill Marta, only to find out she hasn't died. That's because, as Harlan foreshadowed, Ransom didn't know the difference.