Добавьте файлы проекта.

BL/ToSerializ.cs

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+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace GroundOrganizer
+{
+    [Serializable]
+    internal class ToSerializ
+    {
+        public ObservableCollection<PlayGround> PlayGroundList { get; set; }
+    }
+}