If you are still looking for a solution to your problem, Rock elemental need first to use a combat bait (rex arm) and winning the fight before it accept taking sulfur bait (according to default supported dino and bait discussion). You can also check the taming diary.

Thank you. I figured it out and forgot I had this up. I was initially confused because YouTubers didn't show using the trophy bait and fighting it. But all is good now.