Error: 51 - An instance name was not specified while connecting to a Local Database Runtime

Question

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 51 - An instance name was not specified while connecting to a Local Database Runtime. Specify an instance name in the format (localdb)\instance_name.)

Answer 1

in my case I missed the instance_name in the connection string, which was v11.0 for my SQL 2012.