Oracle : Ошибка ORA-12521

"TNS:listener does not currently know of instance 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 in addition to the service name an instance name for an instance
(usually a database instance) 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.
- Wait a moment and try to connect a second time.
- Check which instances are currently known by the listener by executing:
lsnrctl services
- Check that the INSTANCE_NAME parameter in the connect descriptor specifies
an instance name known by the listener.
- Check for an event in the listener.log file.

Попробовать перевести

Поискать эту ошибку на форуме