Problem
ORA-12154: TNS:could not resolve the connect identifier specified
Cause
Problem with the connection configuration or the specified connect identifier.
Possible Resolution
Verify that the TNSNAMES.ORA exists and is accessible.
Make sure that there are no syntax errors in TNSNAMES.ORA.
Verify that the connection string is correct.
Verify if there are any DNS issues.
If the problem is while connect to server using PL sql developer client.try to install SQL developer within Program File isntead of Program Files(x86)'