Swift 3 - Create title bar constraints programmatically

The reason this is happening is because you are using frames. You calculated the frame based on the width of the screen. You do not need frames, you can do this all using auto layout. Instead, you should use constraints to pin your label to it's super view bounds, and give it a static height. for example:

lazy var titleConstraints: [NSLayoutConstraint] = [
    NSLayoutConstraint(item: self.titleLabel, attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0),
    NSLayoutConstraint(item: self.titleLabel, attribute: .right, relatedBy: .equal, toItem: self.view, attribute: .right, multiplier: 1, constant: 0),
    NSLayoutConstraint(item: self.titleLabel, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0),
    NSLayoutConstraint(item: self.titleLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 40)
]

then, in viewDidLoad()

self.view.addConstraints(titleConstraints)

You could simplify your label declaration like so. dont forget the auto resizing mask flag to get constraints to work correctly:

let titleLabel: UILabel = {
    let label = UILabel()
    label.translatesAutoresizingMaskIntoConstraints =  false
    label.backgroundColor = UIColor.blue
    label.text = "Scan Results"
    label.textAlignment = .center
    label.textColor = UIColor.white
    return label
}()

Finally, you are doing strange math to get the top of your view controller to abut the bottom of your nav bar controller. Remove all that garbage and put the following in viewDidLoad() to get the top of your view controller right against the bottom of your UINavigationBar:

self.edgesForExtendedLayout = []

UPDATES:

The problem here is you are appending views and constraints into a View Controller which has not allocated yet.

The reason we append sub views and constraints within viewDidLoad() is because we cannot add subviews and constraints before the view....did....load into memory. Otherwise, it's not there, and you get the error above. Consider breaking out your detailViewController into a class declaration, like so:

class detailViewController: UIViewController {
let eobTitleLabel: UILabel = {
    let label = UILabel()
    label.translatesAutoresizingMaskIntoConstraints =  false
    label.backgroundColor = UIColor.blue
    label.text = "Scan Results"
    label.textAlignment = .center
    label.font = UIFont.boldSystemFont(ofSize: 18)
    label.textColor = UIColor.white
    return label
}()

    lazy var eobTitleConstraints: [NSLayoutConstraint] = [
       NSLayoutConstraint(item: self.eobTitleLabel, attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0),
        NSLayoutConstraint(item: self.eobTitleLabel, attribute: .right, relatedBy: .equal, toItem: self.view, attribute: .right, multiplier: 1, constant: 0),
        NSLayoutConstraint(item: self.eobTitleLabel, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0),
        NSLayoutConstraint(item: self.eobTitleLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 40)
    ]

    override func viewDidLoad() {
        self.view.addSubview(eobTitleLabel)
        self.view.addConstraints(self.eobTitleConstraints)
    }
}

Also, not to come off as offensive, but your code is kind of a mess. Things you should avoid in the future:

1. adding constraints to a label which does not exist. ( rename the label of fix the constraints)
2. you are declaring vars in a outlet method. dont do this, declare methods and properties at the class level.
3. Read about OOP and how it is implemented in swift. This will help you understand the methods and patterns to complete your task :)