Why I'm getting exception ArgumentException: Scene to unload is invalid ? Should I change to UnloadScene without async?

The script is attached to a button in the Inspector to the On Click: The script is attached to both StartGame and StartGameButton:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public class LoadSceneOnClick : MonoBehaviour
{
    public void ActivatePlayer()
    {
        SceneManager.UnloadSceneAsync(0);
    }
}

The exception is on the line:

SceneManager.UnloadSceneAsync(0);

ArgumentException: Scene to unload is invalid
UnityEngine.SceneManagement.SceneManager.UnloadSceneAsync (Int32 sceneBuildIndex) (at C:/buildslave/unity/build/artifacts/generated/common/runtime/SceneManagerBindings.gen.cs:196)
LoadSceneOnClick.ActivatePlayer () (at Assets/My Scripts/Scenes/LoadSceneOnClick.cs:10)
UnityEngine.Events.InvokableCall.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:165)
UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:58)
UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:36)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:45)
UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50)
UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261)
UnityEngine.EventSystems.EventSystem:Update()