Remove me on arrears

I had two accounts with them and asked the settlement amount for the other account which I first had,then continued paying the other account which they deduct as a debit order each month. I then noticed on my credit score that I have arrears and went to the store and spoke with the manager which was unsure whether I missed the payment or not and I showed her my bank statements then she said the issue was on the settlement amount they made a mistake nd when she explains it does not make sense how I got into arrears now my question if ts their mistake why can’t they fix that on credit berau cz now I can’t make credits bcz of them, I left the store without being assisted