ORA-28014: cannot drop administrative users

The below error is encountered when I was trying to drop schema from 12C database.

SQL> drop user BKP CASCADE;
drop user BKP CASCADE
*
ERROR at line 1:
ORA-28014: cannot drop administrative users

Solution :-

SQL> alter session set "_oracle_script"=true;

Session altered.

SQL> drop user BKP CASCADE;
User dropped.