Keyword not supported: 'provider'. Opening SqlConnection

Aleksey Mynkov has it right. But here is more detail since you are needing more clarification.

Your web.config is fine. The auto-generated Visual Studios connection string is using the right setup. Instead, on your webform1 file you need to do 2 things.

1. Add using System.Data.OleDb.OleDbConnection; to the top of your file, and remove the using System.Data.SqlConnection;

2. Change your webform1 code to be:

    private static string conDB = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {
        using (OleDbConnection con = new OleDbConnection(conDB))  //here is the error
        {
        }
    }
   

You should use System.Data.OleDb.OleDbConnection.

I know this is somewhat an old thread and already answered but i'm adding my solution for future reference

I have SQL server 11.0 database, and i encountered the error when i was trying to work with it in SharePoint app, I haven't tried the other proposed answers, but i simply just deleted the "Provider" portion (and reordered) , so my connection string which looked like this:

Provider=SQLOLEDB.1;Password=DBPassword;Persist Security Info=True;User ID=sa;Initial Catalog=DBName;Data Source=DBServer

Now looks like this:

Data Source=DBServer;Initial Catalog=DBName;Persist Security Info=True;User ID=sa;Password=DBPassword;

And it worked just fine