Useless and wont take responsibility for own mistakes

I opened 2 accounts with lewis at Athlone Cape Town. At first i paid my installment at the store and changed in July 2019 to debit order. I did that at the store. Since then, money has been going off my account as expected for the instalment of 2 accounts. I got strange letters from the legal department of lewis saying that i missed payments which i did not. I did not pay too much attention to the letters knowing very well that I am paying my instalments. The balance on the letters kept going up and started to get worried. I went to the store today and asked them about this balance that keeps going up. To my surprise i was told that the head office people did not split the debit order amount to 2 accounts and i am on arrears on the other account, meaning i have not been paying since July when i made the debit order arrangement. That is not fair at all and not only has increased my balance but also ruined my credit score. I am not going to pay money i should not owe. What makes it even worse is that the lady that i was talking to was rude and told me that it is not her fault, she was not there when i opened the account. She told me the branch manager is new in and knows nothing about the accounts. It is not my fault that you did not split the debit order amount to 2 accounts. They are telling me that the only thing they can do is to re-do the debit order and can not do anything about the arrears that are ruining my credit score.