Sometimes a PDB is in a wrong status, like the MIGRATE status if you have just made an upgrade.
The best way to check the status is to execute this query from the CDB
set lines 200
column HOST_NAME format a50
select INST_ID, INSTANCE_NUMBER, INSTANCE_NAME,
HOST_NAME, DATABASE_STATUS, con_id
from gv$instance;
INST_ID INSTANCE_NUMBER INSTANCE_NAME HOST_NAME DATABASE_STATUS CON_ID
---------- --------------- ---------------- -------------------------------------------------- ----------------- ----------
1 1 YD001NFR1 con01db01.mydomain.com ACTIVE 0
2 2 YD001NFR2 con01db02.mydomain.com ACTIVE 0
set lines 200
column name format a30
select INST_ID, CON_ID, NAME, OPEN_MODE, RESTRICTED
from gv$pdbs;
INST_ID CON_ID NAME OPEN_MODE RES
---------- ---------- ------------------------------ ---------- ---
1 2 PDB$SEED READ ONLY NO
1 3 CLML01NFRPLUG READ WRITE NO
2 2 PDB$SEED READ ONLY NO
2 3 CLML01NFRPLUG READ WRITE NO
alter pluggable database CLML01NFRPLUG close instances=('YD001NFR1');
select INST_ID, CON_ID, NAME, OPEN_MODE, RESTRICTED
from gv$pdbs;
INST_ID CON_ID NAME OPEN_MODE RES
---------- ---------- ------------------------------ ---------- ---
1 2 PDB$SEED READ ONLY NO
1 3 CLML01NFRPLUG MOUNTED
2 2 PDB$SEED READ ONLY NO
2 3 CLML01NFRPLUG READ WRITE NO
alter pluggable database CLML01NFRPLUG open instances=('YD001NFR1');
set lines 200
column name format a30
select INST_ID, CON_ID, NAME, OPEN_MODE, RESTRICTED
from gv$pdbs;
INST_ID CON_ID NAME OPEN_MODE RES
---------- ---------- ------------------------------ ---------- ---
1 2 PDB$SEED READ ONLY NO
1 3 CLML01NFRPLUG READ WRITE NO
2 2 PDB$SEED READ ONLY NO
2 3 CLML01NFRPLUG READ WRITE NO
Some more tests by closing one instance and then opening all instances again.
alter pluggable database CLML01NFRPLUG close instances=('YD001NFR1');
alter pluggable database CLML01NFRPLUG open instances=all;
set lines 200
column name format a30
select INST_ID, CON_ID, NAME, OPEN_MODE, RESTRICTED
from gv$pdbs;
INST_ID CON_ID NAME OPEN_MODE RES
---------- ---------- ------------------------------ ---------- ---
1 2 PDB$SEED READ ONLY NO
1 3 CLML01NFRPLUG READ WRITE NO
2 2 PDB$SEED READ ONLY NO
2 3 CLML01NFRPLUG READ WRITE NO
I hope this was helpful.