You may not select this entry type for a preexisting item 6040,591 message On Account Payment

An end user has an existing on account item on a customer.  Another payment was received with no item to apply against.  Users want to put this amount on account under the existing on account item.  The message "You may not select this entry type for a preexisting item 6040,591" is received.  Is there a way to put a second on account amount on the same item using the on account entry type WS-05 system function.  User says they were able to do this previously, but we have been unable to recreate and do the same.  When trying to recreate,