Trying to reverse a GRV

Hello,

Our company recently got Sage Evolution. While trying to recieve an Invoice one of our storekeepers recieved an Invoice with the wrong GRV, to reverse this process I went to the forums and saw a simillar post saying that an RTS is to be used and then allocated to the processed Suppliers Invoice to reverse the error, however I have been sitting in the training module and have tried using an RTS to reverse it but the Purchase Order seems to still be locked even after allocating the RTS to the Suppliers Invoice.

Am I doing something wrong or is there another method that is supposed to be used?

Any and all help is greatly appriciated.