NEXT_DATE must be supplied as a date date, not a string.

Also, it's better practice to name the parameters as you pass them ...

Code:
variable jobno number;

begin
   dbms_job.submit
      (
      jobno =>     :jobno,
      what =>      'trunc_table',
      next_date => NEXT_DAY(TRUNC(SYSDATE), 'MONDAY') + 6/24
      );
   commit;
END;
/