C# console application Invalid Operation Exception

The correct way doing that should be something like:

static void Main(string[] args) {
    string connectionString = "Data Source=H....; 
    Initial Catalog=LANDesk;User ID=Mainstc; Password=xxxxxxxx"; 
    // removed Persist Security Info=True; 


    using(SqlConnection con = new SqlConnection(connectionString))
    {
      if (con.State==ConnectionState.Closed)
      {                      
          con.Open();   
      }
    }


}

Using Using Statement it will automatically dispose your SQL connection.

Check this also: Best Practices for Using ADO.NET on MSDN

Other things to do: Use SQL Management Studio and try to use your sql authentication login credential from your connection string and if you have successfully connected to your database using that account the above code should work for you.

Best Regards

The code should read

using (SqlConnection con = new SqlConnection(connectionString))
{
    con.Open();

    ...
}