Oracle's data buffer cache was deleted using the database command.
"ALTER SYSTEM FLUSH BUFFER_CACHE;"
But then the mistake happened. SQL Error  : ORA-01031.
So I used a different command:
Now this happened: SQL Error  : ORA-00900
The docs say that this command "exec admin.admin_util.flush_buffer_cache;" can be used but it can't. Why? Also, how to do flust_buffer_cache?