Pick over the ideas at https://forums.androidcentral.com/samsung-galaxy-s6-edge-plus/592265-solved-samsung-s6-edge-auto-rotation-setting-disappeared.html
Since there's so many reasons I won't guess which one it is today.
I have the phone listed in the title, purchased new maybe 2-1/2 yrs ago. I'm not a mobile device addict, and I am really not a power user of this tool at all. Most of my use consists of occasional phone call, text message, or email, and photo documentation of stuff I want to look at in more detail later. When I first got the phone, it would auto orient the display for vertical or horizontal format, I assume using internal accelerometer module(s). The phone has seen probably a half-dozen system software updates that I can recall, and it seems to me that the auto-orient function stopped working after an update a few months ago. I don't know enough about the fine points to know if this failure is indeed a mistake in system software update, or a hardware failure, or something I can fix in Settings somewhere. I sure haven't found anything obvious to correct the problem up to now.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic