How do I enable double tap notification bar to sleep/screen off on Galaxy S7 Edge?

Sorry for this question, but I just cannot figure it out. Every time I double tap the notification bar at the top, it does not screen off/sleep, instead it just tries to open the notification window on my S7 Edge. I've been extra careful not to slide my finger trying to get this working. So how do I get Nova Prime to perform screen off when double tapping the notification bar?

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