PLS-00049 bad bind variable when compiling a trigger
you should remove fragment: "USER.". Properly is:

CREATE OR REPLACE TRIGGER TBL_ACTION_DEDUP
BEFORE INSERT
ON ERRORTABLE 
REFERENCING NEW AS NEW OLD AS OLD
FOR EACH ROW
BEGIN  
IF (:OLD.Identifier = :NEW.Identifier) THEN  
  SELECT :OLD.tally + 1 INTO :NEW.tally FROM DUAL;
END IF;  
END TBL_ACTION_DEDUP;
/