试试下面的代码。

override func viewDidLoad() {
        super.viewDidLoad()
       
        let emialTextField: UITextField = {
            let textField = UITextField(frame: CGRect(x: 100, y: 90, width: 100, height: 30))
            textField.borderStyle = .none
            textField.placeholder = "emial adress"
            let bottomline = CALayer()
            bottomline.frame = CGRect(x: 0, y: textField.frame.height + 1, width: textField.frame.width, height: 2)
            bottomline.backgroundColor = UIColor.init(red: 3/255, green: 4/255, blue: 5/255, alpha: 1).cgColor
            textField.layer.addSublayer(bottomline)
            return textField
        }()
        
      
        
        view.addSubview(emialTextField)
        emialTextField.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 100).isActive = true
        emialTextField.topAnchor.constraint(equalTo: view.topAnchor, constant: 90).isActive = true
        emialTextField.heightAnchor.constraint(equalToConstant: 30).isActive = true
        emialTextField.widthAnchor.constraint(equalToConstant: 100).isActive = true
    }

在执行下面的代码时，textField.frame.width似乎为零：

bottomline.frame = CGRect(x: 0, y: textField.frame.height - 2, width: textField.frame.width, height: 2)

因此，您应该通过约束或帧大小为emialTextField设置大小后，将bottomline层添加到文本字段。