Can someone figure out the mistake in the below code cause it's being compiled on Xcode & causing a compiler error here

class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        NSLayoutConstraint.activate([
            sampleView.heightAnchor.constraint(equalToConstant: 50.0),
            sampleView.widthAnchor.constraint(equalToConstant: 50.0),
            sampleView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            sampleView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
            ])
    }
}