In D365, we are seeing an error message during firming of Planned Purchase Orders with Standard marking selected.
The error is referring to table RetailTransactionSalesTrans. 

I can see how firming of a purchase order with Standard Marking would be marking within Dynamics AX in the usual way;  i.e. converting the Pegging against a forward Sales Order transaction to a  Marking/Reservation relationship.

Can anyone explain the flow of firming process through to the RetailTransactionSalesTrans. 

For the record, the error reads like this:
"Cannot edit a record in Sales transactions (RetailTransactionSalesTrans). The operation cannot be completed, since the record was not selected for update"

This could be some timing complexity which we should be able to understand if the process was clarified.