if let clause false when trying to get motion data

I've run into a problem when coding my first iOS app, so please be patient shouldn't I post nonsense or don't understand your answer directly without asking a second time.

I've written this code:

func updateWatch() {
        if motionManager.isAccelerometerAvailable == true {
        if let z: Double = motionManager.accelerometerData?.acceleration.z {
            print("test succeeded)
            }
        } else {
//Timelabel is a UILabel
            timeLabel.text = "Sorry"
            }

        }
    }

updateWatch() is linked to a scheduledTimer with repeat = true (stopwatch) When executing on my iPhone 6s the label says "sorry".

What causes the if let statement to be false and how can I work around it.

Thanks in advance for any help :)