When should translatesAutoresizingMaskIntoConstraints be set to true?

translatesAutoresizingMaskIntoConstraints needs to be set to false when:

1. You Create a UIView-based object in code (Storyboard/NIB will set it for you if the file has autolayout enabled),
2. And you want to use auto layout for this view rather than frame-based layout,
3. And the view will be added to a view hierarchy that is using auto layout.

In this case not all of these are true. Specifically, point 2.

After you return the header view from viewForHeaderInSection it is added to the table view and its frame is set based on the current width of the table view and the height you return from heightForHeaderInSection.

You can add subviews to the root header view (header in your code) and use constraints to layout those subviews relative to the header view.

You have discovered the reason why you can't use autolayout for the header view itself in your comments; at the time you create the view it isn't yet part of the view hierarchy and so you cannot constrain its edges to anything.

In order to have dynamic header sizing, you will need to add subviews to your header view and add constraints between those subviews and header. Then, auto layout can use the intrinsic content size of header to determine the header view size.

Since you are not constraining the frame of header, do not set translatesAutoresizingMaskIntoConstraints to false. You will need to ensure that you have sufficient constraints on your subviews for auto layout to determine the size of header.

You will need a continuous line of constraints from top to bottom and potentially some height constraints for your subviews if the intrinsic content size of that subview is not sufficient.

Any subviews you add to header do need translatesAutoresizingMaskIntoConstraints set to false

You also need to return something from estimatedHeightForHeaderInSection - the closer to your actual header height the better - if you are using tableview.sectionHeaderHeight = UITableViewAutomaticDimension

• For programmatically created view default is true and for views from Interface Builder default is false

  If the property is (or set to) True, the system automatically creates a set of constraints based on the view’s frame and its autoresizing mask. And if you add your own constraints, they inevitably conflict with the autogenerated constraints. This creates an unsatisfiable layout. So When programmatically instantiating views, be sure to set their translatesAutoresizingMaskIntoConstraints property to NO.

With PANKAJ VERMA's answer, it finally makes sense.

In my case, I have an ImageView and only set 2 constraints, yet the error shows that I have more constraints and LayoutConstraints was Unable to simultaneously satisfy constraints:

        view.addSubview(imageView)
        let yConstraint = imageView.centerYAnchor.constraint(equalTo: layout.centerYAnchor)
        yConstraint.identifier = "yConstraint"
        let xConstraint = imageView.centerXAnchor.constraint(equalTo: layout.centerXAnchor)
        xConstraint.identifier = "xConstraint"

Error message:

[LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x600000089360 h=--& v=--& UIImageView:0x7fc1782087b0.minY == 0   (active, names: '|':UIView:0x7fc176406220 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x600000089220 h=--& v=--& UIImageView:0x7fc1782087b0.height == 0   (active)>",
    "<NSLayoutConstraint:0x600000088c30 'UIView-bottomMargin-guide-constraint' V:[UILayoutGuide:0x600001a880e0'UIViewLayoutMarginsGuide']-(34)-|   (active, names: '|':UIView:0x7fc176406220 )>",
    "<NSLayoutConstraint:0x600000088cd0 'UIView-topMargin-guide-constraint' V:|-(48)-[UILayoutGuide:0x600001a880e0'UIViewLayoutMarginsGuide']   (active, names: '|':UIView:0x7fc176406220 )>",
    "<NSLayoutConstraint:0x600000088d70 'yConstraint' UIImageView:0x7fc1782087b0.centerY == UILayoutGuide:0x600001a880e0'UIViewLayoutMarginsGuide'.centerY   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600000088d70 'yConstraint' UIImageView:0x7fc1782087b0.centerY == UILayoutGuide:0x600001a880e0'UIViewLayoutMarginsGuide'.centerY   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

There was basically an almost identical error log for the 2nd constraint ("xConstraint") I set. As you can see in the error logs, I am "over-constraining" my UI. Along with 'yConstraint', I also have 4 other constraints. I believe the constraints are in a tuple, hence the paranthesis around them. XCode tries to be helpful by hinting "(Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)", but personally I don't think this is helpful enough.

What is "Autoresizing Mask"?

I guess its important to know what this means, as this is being converted to constraints, hence the name translatesAutoresizingMaskIntoConstraints. Its an instance property of UIView containing an integer bit mask. It holds onto bits that you can toggle on and off for features like "flexible left margin" and "flexible height" (more of them here).

When a view’s bounds change, that view automatically resizes its subviews according to each subview’s autoresizing mask.

source

So to summarize the Autoresizing Mask, it holds onto the autoresize features you want, like flexible height and width.