ORA-12516, TNS:listener could not find available handler with matching protocol stack

This error message typically indicates that the Oracle listener process is unable to establish a connection to a database instance. There may be several causes for this issue, including:

1. Insufficient resources: The listener process may not have enough available resources to handle incoming connections. This could be due to limited system resources or heavy load on the server.

2. Database instance not available: The database instance that the listener is trying to connect to may not be available or may have been shut down.

3. Incorrect configuration: The listener configuration may be incorrect, preventing it from properly connecting to the database instance.

To resolve this issue, you can try the following steps:

1. Increase resources: If the issue is due to insufficient resources, you can try increasing the available resources on the server.

2. Check database availability: Verify that the database instance is available and running.

3. Check listener configuration: Verify that the listener configuration is correct and that it is able to connect to the database instance.

4. Restart listener: If all else fails, try restarting the listener process and see if that resolves the issue.