Operation not allowed from within a pluggable database Pre-Built Developer VMs for Oracle VM VirtualBox

I downloaded **Pre-Built Developer VMs for Oracle VM VirtualBox** , which contains Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 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.