The error message means that your were not able to reach the SQL Server instance you specified. There can be a number of reasons for this, including:
- The server does simply not exist.
- You spelled the server or instance name wrong.
- SQL Server is not running.
- There are firewalls blocking the way.
Since you don't give any details on why you expect the connection to succeed or what troubleshooting you have done, I can only provide a generic answer.
See further this troubleshooter.