That error message means that you we're not able to access the SQL Server instance you had specified.
Common reasons for this error includes:
- SQL Server not running.
- Server name misspelled.
- You're mistaken on which machine SQL Server runs.
- You have installed a named instance, but you are not given the instance name.
- Firewalls blocking access.
- You are specifying a remote SQL Server instance, but it does not accept remote connections.
- You have not installed SQL Server at all on the machine you specify.
The list is not exhaustive. This troubleshooter can also help you.