What's gone wrong here?

When your code does not compile it can often be a good idea to take a look at the compiler error that Swift generates, they are often more helpful than you might think. You can see them by clicking on the "Preview" button after trying to run your code.

The error generated for your code is this:

swift_lint.swift:33:17: error: argument names for method 'move(direction:)' do not match those of overridden method 'move'
  override func move(direction: String) { 
                ^
                     _ 

Which is essentially just telling you that you don't use the exact same argument names for the overridden move method as the original one does. Which is a requirement when overriding a method.

Specifically you have forgotten to include _ as an external parameter name. If you add that like this:

class Point {
  var x: Int
  var y: Int

  init(x: Int, y: Int) {
    self.x = x
    self.y = y
  }
}

class Machine {
  var location: Point

  init() {
    self.location = Point(x: 0, y: 0)
  }

  func move(_ direction: String) {
    print("Do nothing! I am a machine!")
  }
}

// Enter your code below

class Robot: Machine {
  override func move(_ direction: String) { 
    switch direction {
      case "Up": location.y += 1
      case "Down": location.y -= 1
      case "Left": location.x -= 1
      case "Right": location.x += 1
      default: break
    }
  }
}

Then your code will work.