help on generating a spool

I'm trying to generate a spool but it marks me wrong syntax

when I insert in the variable

COL Nombre_emp new_value Nombre_emp;
SELECT TO_CHAR(TO_DATE(TRUNC(MAX(ID_MES)),'YYYYMM'),'YYYYMM')||'.txt' Nombre_emp FROM estudiante WHERE ID_MES > 201801;

COL Mes new_value Mes;
SELECT MAX(ID_MES) Mes FROM estudiante WHERE ID_MES > 201801;

and insert it into the select with which the spool will be generated

adding the variable in this way

(select * FROM   estudiante WHERE ID_MES = &Mes)

marks me syntax error could you help me