How to use alter system sql statement in AWS RDS oracle

Question

"ALTER SYSTEM FLUSH BUFFER_CACHE;" 

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

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

So I issued a different command:

"The following issue occurred while executing rdsadmin.rdsadmin util.flush buffer cache. Error code [900] [42000] in SQL: ORA-00900

Execute "exec admin.admin util.flush buffer cache;" according to the documentation. Can you explain why I can't use this command despite the fact that it indicates I can?

Does flust buffer cache have any other methods?