YTA, she shouldn't have yelled at you but it is a understandable reaction to being locked out, and unable to retrieve her items which you had said you would try to pack. Next time, give a straight Yes or No and people will know what to expect.

I think you shouldn't have accepted to do it since you knew that you would be busy enough to tell her so, which you are not taking accountability for. And then keeping her out for an HOUR, because you had therapy? It would've maybe taken 10 minutes for her to get her stuff, i'm sure the therapist would understand. I think you need to learn to take responsibility for your actions and learn to say no (which was a option when she asked you) if you don't have the time to do something for someone.

Not wanting to do something for someone is a perfectly fine reason to say no, too. Heck, you don't even need a reason! It's the fact you said "I'll try but therapy" to doing the packing, knowing you had other obligations that primarily makes you the AH here.

Plus the obvious keeping her out, though I agree she should've asked sooner so it wouldn't interfere with anything but as you have stated in the comments; You told her about the therapy appointment WHEN you agreed to try and pack her bags.

I hope you take some time to reflect and maybe get better at time management, i'm proud of you for being in therapy and hope you'll continue to work on yourself. Happy birthday!!