ORA-01031: insufficient privileges ALTER PLUGGABLE DATABASE to OPEN
Just grant SYSDBA/DBA privilege to the user to all containers and login as SYSDBA to perform the task.

Demo:

SQL&gt; create user c##admin identified by c##admin container=all;

User created.

SQL&gt; grant sysdba to c##admin container=all;

Grant succeeded.

SQL&gt; conn c##admin as sysdba
Enter password:
Connected.
SQL&gt; alter pluggable database orclpdb open;