How to add constraints to a view inside a subview of a StackView

Question

In my test program there is a vertical stack view, in its third subview (the green one at the bottom) I want to add a red rectangle at its center. I tried anchoring to centerXAnchor, and centerXAnchor but it doesn't work. I suspect it's because the green view doesn't use X and Y anchor as its size and position are managed by the vertical stack view. What would be the correct way to center the red rectangle inside the green view? Here is my codes and screenshots:

class ViewController: UIViewController {
    @IBOutlet var greenView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()
        let testView = UIView(frame: CGRect(x: 0,y: 0,width: 50,height: 50))
        testView.backgroundColor = .red
        greenView.addSubview(testView)
        greenView.translatesAutoresizingMaskIntoConstraints = false

        testView.centerXAnchor.constraint(equalTo: greenView.centerXAnchor).isActive = true
        testView.centerYAnchor.constraint(equalTo: greenView.centerYAnchor).isActive = true
    }
}

The is the storyboard. The only constraints are First View height = Second View Height, and Green View height = 0.8 StackView Height:

This is the result:

Answer 1

the culprit of this bug is greenView.translatesAutoresizingMaskIntoConstraints = false what I am doing is setting width ,height with Anchor api

let testView = UIView(frame: CGRect(x: 0,y: 0,width: 0,height: 0))
        testView.backgroundColor = .white
        greenView.addSubview(testView)
        testView.translatesAutoresizingMaskIntoConstraints = false

        testView.centerXAnchor.constraint(equalTo: greenView.centerXAnchor).isActive = true
        testView.widthAnchor.constraint(equalToConstant: 50).isActive = true
        testView.heightAnchor.constraint(equalToConstant: 100).isActive = true
        testView.centerYAnchor.constraint(equalTo: greenView.centerYAnchor).isActive = true