System.UnauthorizedAccessException:

I have problem in giving permission to a route to export there a pdf in xamarin android how can I do it: here I have what I have done

//Define permisos requeridos por aplicación.

    readonly string[] PermissionsMyApp =
      {
           Manifest.Permission.WriteExternalStorage,
           Manifest.Permission.ReadExternalStorage
    };
    private void permiso()
    {
        const int RequestExternalStorageId = 0;

        const string permission = Manifest.Permission.WriteExternalStorage;
        if (CheckSelfPermission(permission) == (int)Permission.Granted)
        {
            //Ya se cuenta con permisos!
            exportarPDF();
            return;
        }


        //Se necesita obtener permiso,
        if (ShouldShowRequestPermissionRationale(permission))
        {
            //Explica al usuario porque necesita el permiso.
            AlertDialog.Builder alert = new AlertDialog.Builder(this);
            alert.SetMessage("Permiso requerido para el correcto funcionamiento de la aplicación.").SetTitle("Permiso").SetPositiveButton("OK", delegate { RequestPermissions(PermissionsMyApp, RequestExternalStorageId); }).SetNegativeButton("No", delegate { }).Show();
            return;
        }

        //Finalmente requiere el permiso  permissions and Id
        RequestPermissions(PermissionsMyApp, RequestExternalStorageId);
    }

private void exportPDF ()         {

        /*  PdfDocument document = new PdfDocument();
         PdfDocument.PageInfo pageInfo = new PdfDocument.PageInfo.Builder(597, 820, 1).Create();
         PdfDocument.Page page = document.StartPage(pageInfo);*/



        // create a new document
        PdfDocument document = new PdfDocument();

        /* EditText content2 = new EditText(context);
         content2.Text = "Cat";*/

        // crate a page description
        PdfDocument.PageInfo pageInfo = new PdfDocument.PageInfo.Builder(200, 100, 1).Create();

        // start a page
        PdfDocument.Page page = document.StartPage(pageInfo);

        // draw something on the page
         content2.Draw(page.Canvas);

        // finish the page
        document.FinishPage(page);

        //create the outputstream to sdcard
        var sdCardPath = Android.OS.Environment.ExternalStorageDirectory.AbsolutePath;
        var filePath = System.IO.Path.Combine(sdCardPath, "Médico de familia.pdf");
        // Directory.CreateDirectory(filePath);
        var stream = new FileStream(filePath, FileMode.Create);// me da error de autorizacion pqqqqqq


        // write the document content
          document.WriteTo(stream);

        // close the document
        document.Close();

        Toast.MakeText(ApplicationContext, "PDF Generated", ToastLength.Short).Show();
    }