It looks like you forgot to set translatesAutoresizingMaskIntoConstraints
on the UILabel. By default it will be YES. So the autoresizing mask on that label are translated to additional constraints, which are then conflicting with the one you specified.
Adding this should fix the constraint issue:
label.translatesAutoresizingMaskIntoConstraints = NO;
You should probably think about a vertical constraints on that label as well.