SerializationException: Type ‘UnityEngine.MonoBehaviour’ 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 <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Runtime.Serialization.FormatterServices+<>c__DisplayClass9_0.b__0 (System.Runtime.Serialization.MemberHolder _) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Collections.Concurrent.ConcurrentDictionary2[TKey,TValue].GetOrAdd (TKey key, System.Func2[T,TResult] valueFactory) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Runtime.Serialization.FormatterServices.GetSerializableMembers (System.Type type, System.Runtime.Serialization.StreamingContext context) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo () (at <1f0c1ef1ad524c38bbc5536809c46b48>: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 <1f0c1ef1ad524c38bbc5536809c46b48>: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 <1f0c1ef1ad524c38bbc5536809c46b48>: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 <1f0c1ef1ad524c38bbc5536809c46b48>: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 <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph, System.Runtime.Remoting.Messaging.Header headers) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
I don’t see the ‘LevelCompleted’ class in your source, but I will assume that it derives from MonoBehaviour?
Anyway the exception above says that you are trying to use the .net serializer to serialize a MonoBehaviour, which is not allowed (‘not marked as serializable’ part). Also, the deserialization would be tricky, as you shgould not call its constructor directly (or from the deserializer), but call AddComponent() instead.
Try to create the monoBehaviour externally, and just fill it with deserialized data.
You can also write surrogates for the serializer, a way to provide a custom handling code for the unknown / non-serializable classes. See the .net docs for the serialization.