C:>set ORACLE_SID=ORADEV01 C:>echo %ORACLE_SID%
The echo command should return the SID that you specified in the previous set command. Ensure there are no spaces on either side of the equal sign when you set ORACLE_SID.
If you are providing a service name, ensure that the listener has been started. It is also a good idea to check the sqlnet.log file. This may also give you some indication of what the problem is.
Finally, this error can occur if you created a database with a SID that contains a non-alphanumeric character, e.g., underscore. You will not be prevented from creating a SID with a non-alphanumeric character and the service will start successfully; however, you may see this error when trying to connect to the database.
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