Delete last character in Swift 3

Swift 4 (Addendum)

In Swift, you can apply dropLast() directectly on the String instance, no longer invoking .characters to access a CharacterView of the String:

var runningNumber = "12345"
runningNumber = String(runningNumber.dropLast())
print(runningNumber) // 1234

Swift 3 (Original answer)

I'll assume runningNumber is a String instance. In this case, runningNumber.characters.dropLast() is not of type String, but a CharacterView:

let foo = runningNumber.characters.dropLast()
print(type(of: foo)) // CharacterView

You need to use the CharacterView to instantiate a String instance prior to assigning it back to a property of type String, e.g.

var runningNumber = "12345"
runningNumber = String(runningNumber.characters.dropLast())
print(runningNumber) // 1234

I.e., for your case

@IBAction func onDelPressed (button: UIButton!)  {
  runningNumber = String(runningNumber.characters.dropLast())
  currentLbl.text = runningNumber
}