unable to reverse payment done through online banking

On the 17/01/2018 i accidentally transfered R7000 from my nedbank accout to my FNB beneficiary (luno) without writing the reference number that was provided to me. i wrote my name and surname as the reference , the money is not appearing on my luno account.

i called Nedbank to reverse the payment and they told me that they are unable to reverse the money done using cellphone/internet banking and since the money is already on the account i transferred to.

why do they have internet banking if they are unable to help me reverse my money.

all i want is for them to reverse that money back into my account