Unsuccessful detection of first app launching , how should I do it?

The (Apple) recommended way is to register the default value (as the class UserDefaults implies).

As soon as possible – for example in applicationWillFinishLaunching – insert

let defaultValues = ["isFirstLaunch" : true]
UserDefaults.standard.register(defaults: defaultValues)

The earliest moment is the init method in application delegate

override init()
{
    let defaultValues = ["isFirstLaunch" : true]
    UserDefaults.standard.register(defaults: defaultValues)
    super.init()
}

Then simply write

func viewDidLoad()
    super.viewDidLoad()
    let defaults = UserDefaults.standard
    if defaults.bool(forKey: "isFirstLaunch") {
        defaults.set(false, forKey: "isFirstLaunch")
        let mainStoryboard = UIStoryboard(name: "ViewController", bundle: Bundle.main)
        let vc : ViewController = mainStoryboard.instantiateViewController(withIdentifier: "ViewController") as! ViewController
        self.present(vc, animated: true, completion: nil)
    }
}

The default value is considered until it's changed the first time.