Why can't I SPOOL an EXPLAIN PLAN? [closed]

Why can’t I spool the explain plan to a txt file?

SPOOL EXPLAIN_PLAN_1.txt

set heading off
set echo off
set pages 999
set long 90000

select plan_table_output from
table(dbms_xplan.display('plan_table',null,'typical'));

SPOOL OFF;

Error that I see

SP2-0768: Illegal SPOOL command
Usage: SPOOL { <file> | OFF | OUT }
where <file> is file_name[.ext] [CRE[ATE]|REP[LACE]|APP[END]]

Oracle Version: 11g