无法添加编程布局约束

Question

我正在使用库DLRadioButton创建一组单选按钮，并希望该组居中，就在ViewController的底部边缘之外。

我正在尝试使用程序化约束，以便确保在每台设备上正确呈现无线电组：

 var buttons = [DLRadioButton]()

    for x in 0..<self.numberOfChildren {
    let answerLabel = snapshot.childSnapshot(forPath: "answers").childSnapshot(forPath: String(x+1)).childSnapshot(forPath: "answer").value
            let firstRadioButton = self.createRadioButton(frame: CGRect(x: self.view.center.x, y: CGFloat(x)*32, width: 100.0, height: 120.0), title: answerLabel as! String, color: UIColor.green)
            firstRadioButton.tag = x
            buttons.append(firstRadioButton)
            self.view.addSubview(firstRadioButton);
        }

    let groupButtons = DLRadioButton()
    let margins = self.view.layoutMarginsGuide
    **groupButtons.bottomAnchor.constraint(equalTo: margins.bottomAnchor, constant: 20).isActive = true**
    groupButtons.otherButtons = buttons
    })