What would be causing the following warning (and subsequent alignment issues on iOS 7)?
Attribute Unavailable: Layout attributes relative to the layou
I'm currently in the same situation as I'm creating an app in Xcode 6 that targets iOS7 and 8. That warning appears because the constraints relative to the margin aren't available in iOS7, but they are created by default in Xcode 6.
You can absolutely go back and edit the constraint to not use that 'Relative to margin' feature as suggested by the other answers to this question.
When creating new constraints, I hold down the Option key when choosing which kind of constraint to create. This gives me the ability to create a constraint that's not based on the margin right away so I don't have to go back and fix it afterwards.