How to change the background color of UIStackView?

You can’t do this – UIStackView is a non-drawing view, meaning that
drawRect() is never called and its background color is ignored. If you
desperately want a background color, consider placing the stack view
inside another UIView and giving that view a background color.

Reference from HERE.

EDIT:

You can add a subView to UIStackView as mentioned HERE or in this answer (below) and assign a color to it. Check out below extension for that:

extension UIStackView {
    func addBackground(color: UIColor) {
        let subView = UIView(frame: bounds)
        subView.backgroundColor = color
        subView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        insertSubview(subView, at: 0)
    }
}

And you can use it like:

stackView.addBackground(color: .red)