Can I connect to Oracle RDS root container

    When you create an Amazon RDS for Oracle instance using the multitenant architecture (CDB), you cannot connect to the root container (CDB) level. Amazon RDS for Oracle restricts access to the CDB and only allows connections to the tenant databases (PDBs).

    This is a limitation of the RDS for Oracle implementation of the multitenant architecture. You always connect to the tenant database (PDB) rather than the CDB. When connecting, you need to specify the endpoint for the PDB just as you would for a non-CDB database, with the only difference being that you specify the PDB name for the database name.

    Additionally, Amazon RDS for Oracle multi-tenant does not allow the creation of common users, which would typically be created at the CDB level. You can only connect to individual PDBs using either the corresponding primary user or other local users created on that tenant database.

    If you need direct access to the CDB level and the ability to create common users, you might want to consider Amazon RDS Custom for Oracle instead, which provides access to built-in Oracle users like SYS and SYSTEM and grants more administrative privileges.
  

Sources
About RDS's oracle cdb issue | AWS re:Post
Get started with the Multi-Tenant feature of Amazon RDS for Oracle | AWS Database Blog
Privileged access - AWS Prescriptive Guidance
Oracle RDS Architecture | AWS re:Post