Unable to get to Rune Challenge in Mission 6

[rampage7]

I am playing Doom 2016 Mission 6 at the Hurt Me Plenty Level.  I have just completed and reached the checkpoint for the first exercise in the mission, a big battle scenario.  I have played through nearly all of Mission 7 on two previous efforts to get through the game.  (I had to twice discard a game disc because the screen froze at nearly the same point in Mission 7 each time and the game gave me an error message that the disc was damaged corrupted.)  I am using for assistance a YouTube video guide prepared by a gamer identified as Maka91.  On the video he says he is playing at the first level of game difficulty.)  On his video at this point in game he is able to proceed directly through an open gate to a Rune Challenge.  I have not been able to get to the Rune Challenge in my efforts to get through the Mission because, even though I have eliminated all the enemies in the exercise at this point, the gate in front of the Rune Challenge cave remains closed.  The gate remains closed even after I go to a small sarcophagus in a cave at the checkpoint and turn a knob on it, thinking this would unlock the Rune Challenge gate.  I viewed two other YouTube videos of this mission and both of these players had the same problem I did in that they were also unable to get to the Rune Challenge and had to find another way to proceed with the mission.  I used these other videos to proceed with the mission until I was able to catch up to the Maka91 video further on in the mission.  I am therefore able at this point to proceed with the mission but I would like to do the Rune Challenge.  Is there something I am missing that  I could do to be able to get to the Rune Challenge?

[RonnieJamesDiner]

If I'm remembering correctly, it should open when you open the blue skull door.