Hi.
The RectangleShape is not shown beause it is not inside your view.
It's position is 0,0 with a size of 100, 100 and your view starts at position 100, 100 with a size of 400, 200. So it is outside your view.
You have to move the rect inside the view e.g:
rect.setPosition(sf::Vector2f(100, 100));