You could tell your supervisors that you have hard time with certain tasks and need an easier load. They might need somebody just doing stock. There’s no reason you can’t ask them.
I’d not complain that you aren’t getting “proper” training as it’s hard to define proper. In my experience retail doesn’t provide very extensive training. Especially if you are seasonal employee.
It’s possible that you do need longer training than they anticipated but that’s why you’d benefit from working with vocational disability services. They’ll help with employment that suits your needs and they also help with job skills. Have you ever contacted them? I provided you with their info more than ones
Retail could be brutal. Their goal is to make a profit. They can’t afford long training or people take extra breaks etc as retail management is under huge pressure. But that’s why it would benefit you to seek employment though disability services