How Do I Resolve This Issue?

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C#3.5

SQL-Server-2008R2

public void GetBackup(string strDBName)
{
    if (string.IsNullOrEmpty(strDBName))
    {
        MessageBox.Show("Server Name & Database can not be Blank");
        return;
    }
    else
    {                
        string destinationPath = ConfigurationManager.AppSettings.Get("BackupDestinationPath");
        

BackupDeviceItem deviceItem = new BackupDeviceItem(destinationPath, DeviceType.File);
            
          
             ServerConnection connection = new ServerConnection(ConfigurationManager.ConnectionStrings["CostAllocationEntities2"].ConnectionString);               
           
            Server sqlServer = new Server(connection);                
            Database db = sqlServer.Databases[strDBName];

The above line Database db = sqlServer.Databases[strDBName]; is causing the below mentioned error.

Error : "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: 26 - Error Locating Server/Instance Specified)"}

Message : Failed to connect to server Data
Source=PC210090\\SQLEXPRESS;Initial Catalog=testdb2;Persist Security
Info=True;User ID=sa;password=password-1"}