The TCP/IP protocol must be enabled in the instance that you are trying to connect to.
In the SQL Server Configuration Manager, in the list of network protocols for the instance, the status for TCP/IP must be set to "Enabled".
Restart the SQL Server instance if it is already running, along with the SQL Server Browser service (if you use this at your site).
Some versions of SQL Server do not allow remote connections, by default, which means that the default setting for TCP/IP is "Disabled".