Cannot deserialize JSON to new instances of type 'X'

I’m making a menu of options, and I want the settings made to be saved in .json format, that does not give any errors, but when I start the game and I want to load them, it says:

Can not deserialize JSON to new instances of type ‘X’

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using System.IO;

public class SettingManager : MonoBehaviour {
	public Toggle fullscreenToggle;
	public Dropdown resolutionDropdown;
	public Dropdown textureQualityDropdown;
	public Dropdown antialiasingDropdown;
	public Dropdown vSyncDropdown;
	public Slider musicVolumeSlider;
	public Button applyButton;

	public AudioSource musicSource;
	public Resolution[] resolutions;
	public GameSettings gameSettings;

	void OnEnable()
		gameSettings = new GameSettings();

		fullscreenToggle.onValueChanged.AddListener(delegate { OnFullscreenToggle(); });
		resolutionDropdown.onValueChanged.AddListener(delegate { OnResolutionChange(); });
		textureQualityDropdown.onValueChanged.AddListener(delegate { OnTextureQualityChange(); });
		antialiasingDropdown.onValueChanged.AddListener(delegate { OnAntialiaingChange(); });
		vSyncDropdown.onValueChanged.AddListener(delegate { OnVSyncChange(); });
		musicVolumeSlider.onValueChanged.AddListener(delegate { OnMusicVolumeChange(); });
		applyButton.onClick.AddListener (delegate { OnApplyButtonClick(); });

		resolutions = Screen.resolutions;
		foreach(Resolution resolution in resolutions)
			resolutionDropdown.options.Add(new Dropdown.OptionData(resolution.ToString()));
		if (File.Exists(Application.persistentDataPath + "/gamesettings.json") == true)

	public void OnFullscreenToggle()
		gameSettings.fullscreen = Screen.fullScreen = fullscreenToggle.isOn;

	public void OnResolutionChange()
		Screen.SetResolution(resolutions[resolutionDropdown.value].width, resolutions[resolutionDropdown.value].height, Screen.fullScreen);

	public void OnTextureQualityChange()
		QualitySettings.masterTextureLimit = gameSettings.textureQuality = textureQualityDropdown.value;

	public void OnAntialiaingChange()
		QualitySettings.antiAliasing = gameSettings.antialiasing = (int)Mathf.Pow(2f, antialiasingDropdown.value);

	public void OnVSyncChange()
		QualitySettings.vSyncCount = gameSettings.vSync = vSyncDropdown.value;

	public void OnMusicVolumeChange()
		musicSource.volume = gameSettings.musicVolume = musicVolumeSlider.value;

	public void OnApplyButtonClick()

	public void SaveSettings()
		string jsonData = JsonUtility.ToJson(gameSettings, true);
		File.WriteAllText(Application.persistentDataPath + "/gamesettings.json", jsonData);

	public void LoadSettings()
		gameSettings = JsonUtility.FromJson<GameSettings>(File.ReadAllText(Application.persistentDataPath + "/gamesettings.json"));

		musicVolumeSlider.value = gameSettings.musicVolume;
		antialiasingDropdown.value = gameSettings.antialiasing;
		vSyncDropdown.value = gameSettings.vSync;
		textureQualityDropdown.value = gameSettings.textureQuality;
		resolutionDropdown.value = gameSettings.resolutionIndex;
		fullscreenToggle.isOn = gameSettings.fullscreen;

I came across the same problem. You simply forgot to delete the monobehavior from gamesettings.
open the file and make sure it read

public class GameSettings {

you probably have the standard ending " : Monobehavior { " but JSON cannot read monobehavior class and this is what it is trying to tell you. they only read class.

@haruna9x |This script I’ve taken from the internet, and I do not understand what I should do, I’ve tried it but it gives me errors. Can you explain it a little better?

Sorry for my bad English, I’m from Spain.