Xamarin android save text file

i am using xamarin and i want to save txt file there so i have some script that is saving

    private void SavetoSd()
           {
               var sdCardPath = Android.OS.Environment.ExternalStorageDirectory.Path;
               var filePath = System.IO.Path.Combine(sdCardPath, "iootext.txt");
               if (!System.IO.File.Exists(filePath))
               {
                   using(System.IO.StreamWriter write = new System.IO.StreamWriter(filePath,true))
                   {
                       write.Write(etSipServer.ToString());
                   }
               }

           }

but the error is that

System.UnauthorizedAccessException: Access to the path "/mnt/sdcard/iootext.txt" is denied.
any help??

in manifest i have added this

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />