Can’t get update one prompt for same variable over multiple tables

Your query and the results don't match, but you're probably right.

This will return the actual available space (modify it accordingly for TBs or 1024**3):

SELECT DatabaseName, 
   CAST(SUM(    MaxPerm                  / 1000**3) AS DEC(9,2)) AS MaxPermGB, 
   CAST(SUM(CurrentPerm                  / 1000**3) AS DEC(9,2)) AS CurrentPermGB, 
   CAST(MAX(CurrentPerm) * (HASHAMP()+1) / 1000**3  AS DEC(9,2)) AS SkewedPermGB, -- due to skewed tables
   MaxPermGB - SkewedPermGB AS AvailablePermGB -- this space can be assigned to other databases
FROM dbc.DiskSpaceV
WHERE MaxPerm > 0
GROUP BY 1
ORDER BY AvailablePermGB DESC

MODIFY DATABASE will only work if enough perm is available in the parent/immediate owner. If this database is not in the 2nd level of the hierarchy dbc is not the direct owner and then it's more complicated. Now you better use Tools -> Move Space, but this might fail if you don't have appropriate access rights.

First of all check whether your pluggable database is opened or not.

SQL> select name, open_mode from v$pdbs where name='PDBORCL';

If it is in mounted mode then,

SQL> alter pluggable database pdborcl open;

Now on SQL Developer Create new connection as,

Connection Name: demo-pdborcl

Username: demo

Password: *****

Connection Type: Basic(If you wish to use TNS connection type you have to create TNS Network Alias in client side.)

Hostname: ip/dns

Port: 1521

SID: (If you use SID like 'ORCL' you will be connected to root container, therefore use Service name instead of SID)

Service name: (You can get your pdb's service name by firing $ lsnrctl serivce, also you can use USE_SID_AS_SERVICE_listener parameter on $ORACLE_HOME/network/admin/listner.ora file as USE_SID_AS_SERVICE_listener=on and any given SID will be treated as service name.

Some useful links-