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I am trying to code the label in the viewDidLoad function. The label is showing up but the code I am using now is not affecting the positioning. I am trying to code everything in the viewDidLoad function. You can see the screenshot below.

override func viewDidLoad() 
 super.viewDidLoad()

 let backbutton = UILabel()
 backbutton.backgroundColor = UIColor.orange
 backbutton.translatesAutoresizingMaskIntoConstraints = false
 backbutton.widthAnchor.constraint(equalToConstant: 300).isActive = true
 backbutton.heightAnchor.constraint(equalToConstant: 300).isActive = true
 backbutton.centerXAnchor.constraint(equalTo: backbutton.centerXAnchor, constant: 100).isActive = true
 backbutton.centerYAnchor.constraint(equalTo: backbutton.centerYAnchor, constant: 300).isActive = true
 view.addSubview(backbutton)

This is how you can align the label into the center of the view,

let backbutton = UILabel()
view.addSubview(backbutton)

backbutton.backgroundColor = UIColor.orange
backbutton.translatesAutoresizingMaskIntoConstraints = false
backbutton.widthAnchor.constraint(equalToConstant: 300).isActive = true
backbutton.heightAnchor.constraint(equalToConstant: 300).isActive = true
backbutton.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 0).isActive = true
backbutton.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 0).isActive = true

When you are setting constraints programmatically, make sure you add the view into the super/parent view before applying constraints. Secondly in the below lines, you are telling the backButton label to align its center to itself (i.e, backbutton.centerXAnchor.constraint(equalTo: backbutton.centerXAnchor).

backbutton.centerXAnchor.constraint(equalTo: backbutton.centerXAnchor, constant: 100).isActive = true
backbutton.centerYAnchor.constraint(equalTo: backbutton.centerYAnchor, constant: 300).isActive = true

As you want to align it center vertically and horizontally to its parent view so you should set the center constraints equal to parent view as below,

backbutton.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 0).isActive = true
backbutton.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 0).isActive = true

Try this!!

 self.view.setNeedsUpdateConstraints()
 self.view.layoutIfNeeded()

Try using CGRect instead:

 let screenWidth = UIScreen.main.bounds.width
 let screenHeight = UIScreen.main.bounds.height
 let xPostion:CGFloat = screenWidth - 150 //150 is half of view's width
 let yPostion:CGFloat = screenHeight - 150 //150 is half of view's height
 let buttonWidth:CGFloat = 300
 let buttonHeight:CGFloat = 300
 backbutton.frame = CGRect(x:xPostion, y:yPostion, width:buttonWidth, height:buttonHeight)

instead of screen width/height you can also try to get the width of your super view using:

 self.view.frame.width 
 self.view.frame.height