ORA-65054: Cannot open a pluggable database in the desired mode

In RAC environment, we cannot open PDB with the following messages:

SQL> alter pluggable database open instances=all;

*
ERROR at line 1:
ORA-65054: Cannot open a pluggable database in the desired mode.

Check CDB still in Mount status:

SQL> alter session set container=CDB$ROOT;

Session altered.

SQL> show con_name

CON_NAME
——————————
CDB$ROOT

QL> select name,open_mode from v$database;

NAME    OPEN_MODE
------  ---------
TESTDB  MOUNTED


SQL> alter database open ;

*
ERROR at line 1:
ORA-65054: Cannot open a pluggable database in the desired mode.