Explain plan works in sqlplus but not in pl/sql

Explain plan works in sqlplus:

SQL> explain plan set statement_id = '1ng6v39qb5hxg' into my_plan_table for /*
  2    The name + type results of these queries will be used by the Code Assistant
  3    if the "Describe Context" option is enabled. After typing 3 or more characters
  4    the Code Assistant will show a list of matching names.
  5    Separate multiple queries with semi-colons and use the :schema bind variable
  6    to restrict names to the currently connected user.
  7    In case of an error the query results will be omitted. No error message will
  8    be displayed.
  9    Place this file in the PL/SQL Developer installation directory for all users,