This form of the ORA-12541 error commonly happens when the database or the listener processes are in the middle of a startup, or when the database (mysid in your case) has not been registered with the listener.
Have a look at these to understand when and why such error occurs and how to handle it:
ORA-12514: TNS:listener does not currently know of service
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
More detail here:TNS:listener does not currently know of service requested in connect descriptor
], it says:
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.
- 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.