Is it correct to convert from computed property to lazy variable under constant struct?

Question

I wanted to change my code to use lazy variable instead of Computed property.
this code works

struct Point {
    var x = 0.0, y = 0.0
}
struct Size {
    var width = 10.0, height = 10.0
}
struct Rect {
    var origin = Point()
    var size = Size()
    var center: Point{
        get{
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        }
    }}
let shape = Rect()
print(shape.center)

But this code produces an error

struct Point {
    var x = 0.0, y = 0.0
}
struct Size {
    var width = 10.0, height = 10.0
}
struct Rect {
    var origin = Point()
    var size = Size()
    lazy var center = Point(x: origin.x + (size.width / 2), y: origin.y + (size.height / 2)) 
}
let shape = Rect()
print(shape.center)  

I can fix the error if I changed let shape = Rect() to variable type var shape = Rect() OR when I change Rect to class instead of struct
What is wrong with my code and why does it give me error?