LATE BUT ANSWER FOR ME was just dragging and dropping the Camera Actor that was missing into the blank/missing Cut box.
At some point in my messing with creating keyframes and adjusting parameters, I had simply slipped up somewhere and ticked off the LockView or dragged something to another spot by mistake. I simply did Undo until I felt safe, and dragged the CameraActor that should’ve been in the missing box in it. Everything else worked and none of the previous keyframe moves I had made were affected.