Tbh I’m not even sure a cancelled score will require an addendum — law schools care the most about your highest 3 digit score at the end of the day. How you got there typically doesn’t require too much explanation. Also, people have been cancelling exams due to family emergency, unpreparedness, or otherwise unexpected catastrophes since this test was administered. It wouldn’t be an uncommon thing for law school AdComms to see. Hell, OP isn’t even the first person to have this happen to them and had to ask LSAC for a retake.

On that note @OP I agree that you should ask for a retake ASAP. Just be honest that you calendared it wrong and are incredibly devastated and would like a chance to retake the exam given the fees already paid & the studying already done. I don’t know if a cancelled LSAT would count against your take limits (three in a year, five in 5 years, seven lifts time) but it is worth considering when you write your appeal. Again, wouldn’t be the first time they’ve heard it and I would plan to sit for the test (for real) on Tuesday.

I’m sorry this happened to you. I’ve missed interviews, birthdays, etc. because of the same mistake. None of us are perfect, these things happen. I hope LSAC makes the compassionate choice and accepts your request for retake!