I couldn't find a way to record slow motion video. It was said that there should be a 'Recording mode' option in camera settings but no such item. I installed third party camera programs, they don't have slow motion options either.
If the ability were there it would be one of your camera modes to select from. The fact even a 3rd party app can't do it had me thinking it may be a hardware limitation so I looked your phone up.
GSMArena which is usually very accurate on specifications lists the Exynos 850 for it's SOC and only shows 1080p@30 FPS for camera video options.
Looking up the Exynos 850 on Samsungs website it only lists FHD@60 FPS for video.
I don't think your phone has the hardware necessary for slow motion.