Literally anything besides just “My greatest challenge was the MCAT, getting a bad score was the worst thing ever to happen to me”. there has to be something outside of academics. Like you could write about how a challenge affected your relationships with loved ones in your life, struggles doing well at your job, or just surviving and trying to pay the bills, and incidentally you could add that it also made studying for the MCAT hard for one line. That might be the extent to which it’s okay to discuss the MCAT in this kind of essay.
There are many posts and words of advice written for applicants who have considered writing about a bad grade or test score that will explain it far better than I can. You can easily find this searching on google, Reddit, and SDN.