You are on probation as a new hire. You will be on probation for the first 60-90 days. They can terminate you for no reason.
After you get past probation period, they should not be able to fire you for ring tender. They can threaten all they want but the union will tell them no.
You have been there 30 days. It is time to take the training wheels off and pick up the pace. Very important to not miscount money. So, keep that in mind. Ask other cashiers for tips on improving your ring tender.
Just go to Walmart if they fire you. Their cashiers do like 4 to 6 items per minute lol, and I've seen the same f*ckin slobs have been there for years whenever I go shopping so obviously their management doesn't care. Plus they don't even have to do that kroger card nonsense, and they get 10 an hour starting. Hell, you should think of applying there and quitting that sh*thole once you're in lol.