It is also possible that the database has not registered with the listener. Try restarting the database and try connecting again to see whether this resolves the problem. Note that it can take 60 seconds for a database to register with a listener.
Finally, check the sqlnet.ora file to determine whether NAMES.DIRECTORY_PATH = (HOSTNAME). Comment this line out and try connecting again. When this parameter is set to HOSTNAME, SQL*Plus is using only the hostname to attempt to identify the remote database SID and resolve the address.
Dig Deeper on Oracle database installation, upgrades and patches
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