ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
 *Cause:  The listener received a request to establish a connection to a
 database or other service. The connect descriptor received by the listener
 specified a service name for a service (usually a database service)
 that either has not yet dynamically registered with the listener or has
 not been statically configured for the listener.  This may be a temporary
 condition such as after the listener has started, but before the database
 instance has registered with the listener.
 *Action:
  - Wait a moment and try to connect a second time.
  - Check which services are currently known by the listener by executing:
    lsnrctl services <listener name>
  - Check that the SERVICE_NAME parameter in the connect descriptor of the
    net service name used specifies a service known by the listener.
  - If an easy connect naming connect identifier was used, check that
    the service name specified is a service known by the listener.
  - Check for an event in the listener.log file.

ORA-12154 ALWAYS only occurs on SQL Client & no SQL*Net packets ever leave client system

ORA-12514 ALWAYS only occurs due to a problem on DB Server system.

my week has started magnificently ...

orcl> alter session set nls_territory='America';

Session altered.

orcl> select to_char(sysdate,'d') from dual;

T
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2

orcl> alter session set nls_territory='United Arab Emirates';

Session altered.

orcl> select to_char(sysdate,'d') from dual;

T
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3

orcl>

orcl> alter session set nls_territory='Croatia'

Session altered.

orcl> select to_char(sysdate,'d') from dual;

T
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1