Constrain to margin isn't working properly on Xcode

I was having the same problem and recently resolved the issue by turning off the safe area. Safe Area Layout is enabled by default. To disable Safe Area Layout in Xcode 9, choose View > Utilities > Show File Inspector and deselect the checkbox for Use Safe Area Layout Guides. This should correct the issue. Good luck, hope it helps.

Check to see if your UIImageView is overlapping or placed past that margin line. If this is the case, the constrain to margin thing doesn't do anything. What I did to fix this was manually move that object so that it was away from the margin line and try again.

This lets you keep the Safe Area.