Purchase Order Receipt (P4312). Is it possible to associate a reason code/message during Receipt Rev

No, currently there is no way to add a reason code/associated text during a Receipt Reverse (option 8). Detail fields and Associated Text (option 3) are disabled during a Receipt Reverse. You can add a reason code/associated text while entering a Receipt (option 1), closing a line (option 7) or cancelling a line (option 9).
The information that you enter the Reason Code field (RCD)  is recorded in the Purchase Order Receiver file (F43121), but currently it is not printed in any of our reports.
Associated Text is recorded in the Sales Order/Purchasing Text Detail Lines  file (F4314).
Associated Text can be printed by Print Purchase Orders (P43500) and by Print Purchase Receivers (P43510), when associated text processing option is enabled.