Cannot connect to PDB

Hi,

On 19 c on AIX

cannot connect to PDB:

sqlplus system@PDB

Enter password:

ERROR:

ORA-12154: TNS:could not resolve the connect identifier specified

But connected to cdb I have:

SQL> SELECT name, open_mode from v$pdbs;

NAME         OPEN_MODE

-------------------- ------------------------------

PDB$SEED       READ ONLY

PDB       READ WRITE

And:

tnsping PDB

Used parameter files:

/sqlnet.ora

TNS-03505: Failed to resolve name

I tried also:

sqlplus system@hostname/PDB

ORA-12154: TNS:could not resolve the connect identifier specified

Thanks.