La query que deberia ser

SELECT cuenta, vencida ,enCurso as val, IFNULL(lc.razon_social, 'S/I') as cliente
FROM tbl_resumen_informe_gestion
LEFT JOIN (select distinct(t.deudor), t.razon_social from tbl_linea_credito t WHERE t.area = 'C101' or t.area = 'C103') lc
ON tbl_resumen_informe_gestion.cuenta = lc.deudor
WHERE (vencida <> 0 or enCurso<>0) AND mes=9 AND año=2018
and clave=1207 and ejecutivo='Milva Ortiz' order by cuenta