how i solve mutation

i create this trigger but mutation error occurred,how i solve it ? 

 create or replace trigger po.roy_trg_seq

  after insert  on po.RCV_TRANSACTIONS

  for each row

declare

  -- local variables here

begin

  if :new.transaction_type = 'DELIVER' and :new.organization_id = 83 then

    update po.rcv_transactions

       set attribute1 = (select nvl(max(nvl(attribute1, 0)), 0) + 1

                           from po.rcv_transactions

                          where transaction_type = 'DELIVER'

                            and organization_id = 83)

     where TRANSACTION_ID = :new.TRANSACTION_ID;

   

  end if;

end roy_trg_seq;