I followed the Brackeys save tutorial and it won't work

This is the error that shows when I try to save.

SerializationException: Type 'UnityEngine.ScriptableObject' in Assembly 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable. System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers (System.RuntimeType type) (at :0) System.Runtime.Serialization.FormatterServices+<>c_DisplayClass9_0.b0 (System.Runtime.Serialization.MemberHolder ) (at :0) System.Collections.Concurrent.ConcurrentDictionary`2[TKey,TValue].GetOrAdd (TKey key, System.Func`2[T,TResult] valueFactory) (at :0) System.Runtime.Serialization.FormatterServices.GetSerializableMembers (System.Type type, System.Runtime.Serialization.StreamingContext context) (at :0) System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo () (at :0) System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize (System.Object obj, System.Runtime.Serialization.ISurrogateSelector surrogateSelector, System.Runtime.Serialization.StreamingContext context, System.Runtime.Serialization.Formatters.Binary.SerObjectInfoInit serObjectInfoInit, System.Runtime.Serialization.IFormatterConverter converter, System.Runtime.Serialization.Formatters.Binary.ObjectWriter objectWriter, System.Runtime.Serialization.SerializationBinder binder) (at :0) System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize (System.Object obj, System.Runtime.Serialization.ISurrogateSelector surrogateSelector, System.Runtime.Serialization.StreamingContext context, System.Runtime.Serialization.Formatters.Binary.SerObjectInfoInit serObjectInfoInit, System.Runtime.Serialization.IFormatterConverter converter, System.Runtime.Serialization.Formatters.Binary.ObjectWriter objectWriter, System.Runtime.Serialization.SerializationBinder binder) (at :0) System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize (System.Object graph, System.Runtime.Remoting.Messaging.Header[] inHeaders, System.Runtime.Serialization.Formatters.Binary.__BinaryWriter serWriter, System.Boolean fCheck) (at :0) System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph, System.Runtime.Remoting.Messaging.Header[] headers, System.Boolean fCheck) (at :0) System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph, System.Runtime.Remoting.Messaging.Header[] headers) (at :0) System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph) (at :0) SaveSystem.SavePlayer (PlayerSystem player) (at Assets/Scripts/SaveSystem.cs:15) PlayerSystem.SavePlayer () (at Assets/Scripts/Player/Rigidbody Movement/PlayerSystem.cs:189) UnityEngine.Events.InvokableCall.Invoke () (at :0) UnityEngine.Events.UnityEvent.Invoke () (at :0) UnityEngine.UI.Button.Press () (at C:/Program Files/Unity/Hub/Editor/2020.2.1f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Button.cs:68) UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/Program Files/Unity/Hub/Editor/2020.2.1f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Button.cs:110) UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/Program Files/Unity/Hub/Editor/2020.2.1f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/ExecuteEvents.cs:50) UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at C:/Program Files/Unity/Hub/Editor/2020.2.1f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/ExecuteEvents.cs:262) UnityEngine.EventSystems.EventSystem:Update() (at C:/Program Files/Unity/Hub/Editor/2020.2.1f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/EventSystem.cs:385)