Can a head cashier do this?

(For context, I play football, and my teams fall training camp has just begun, and as such I am quite sore and have blisters on my feet) I was in self checkout today, sitting on a stool that was left there by whoever was there the previous night. I work part time and I am on opening shift. I had restocked all the food and drinks already, so I sat on the stool because my feet are sore and covered in blisters. My manager came over and made me get off the stool (of which we have 3 more of in a close) and I watched her go and sit on it herself— she said it was because I don’t have a disability. She said she can’t make an exception for temporary injuries.

is there a way for me to get my stool back?

-when I say manager I meant head cashier