ERROR IN ORACLE

I'm trying to make a trigger in sql, but it gives me three errors that I can not solve, you can tell me if you see any error in this trigger.

CREATE OR REPLACE TRIGGER DL_3
BEFORE INSERT ON CONGRESOS FOR EACH ROW
BEGIN

  SELECT AC.id_articulo
  FROM articulos, arti_congresos
  WHERE articulos.nomcongreso = arti_congresos.nomcongreso

  IF(A.nomcongreso <>: NULL) THEN RAISE_APPLICATION_ERROR(-20005, 'El articulo ya esta publicado')  END IF
  ELSE IF (id_articulo <>: arti_congresos.id_articulo AND sysdate <>: congresos.fecha_ini) 
      THEN RAISE_APPLICATION_ERROR(-20007, 'El articulo está presentado en un congreso en la fecha actual') END IF 
END

I get an error

  

sql statement ignored

on the FROM articulos, arti_congresos line, I do not understand anything.