The message is very clear, your connection string is not correct to connect to your sql server database.
Possibly you are using an incorrect host name or instance name. For example SQL EXpress usually installs with an instance name of SQLExpress, so if it is on your local machine you would specify the server=localhost\SQLExpress or on another server it would be serverhostname\SQLExpress
Or possibly your SQL server is not configured to allow remote connections.
Hope it helps,