How to use alter system sql statement in AWS RDS oracle

Question

Oracle's data buffer cache was deleted using the database command.

"ALTER SYSTEM FLUSH BUFFER_CACHE;" 

But then the mistake happened. SQL Error [1031] [42000]: ORA-01031.

So I used a different command:

"EXEC rdsadmin.rdsadmin_util.flush_buffer_cache; 

Now this happened: SQL Error [900] [42000]: 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?