Debit order reversal

I have been trying to reverse a debit order from April and cannot do it at their APP since it says debit order cannot be reversed. I spoke to a banker through secured chat and she advised that I have to call *** that's goes to voice prompt that advised a debit order reversal is unavailable. How can such a big institution that we trust with our money doesn't have a systems to reverse unauthorised debit orders. I'm so much disappointed and thinking of taking my business to another institution.