I downloaded Pre-Built Developer VMs for Oracle VM VirtualBox , which contains Oracle Database 12c Enterprise Edition Release 184.108.40.206.0 I want to BACKUP the Database using Rman, but it is in NOARCHIVELOG mode, so it is necessary to shutdown and change the database to ARCHIVELOG mode, but when I try to alter database to archive log , it gives an error saying operation not allowed from within a pluggable database. An operation was attempted that can only be performed in the root container.
"Pluggable Databases" are not really Oracle Databases at all.
Think of them more like "TableSpaces on Steroids".
All the "real" database work, including Redo Logging, is done by the Container Database (CDB). Seems a bit weird, I know, but that's how Our Friends in Redwood City decided to do things.
You can do just about everything that PDBs do using Tablespaces, Schemas and [tight] Security Management (and we used to just that!)