You need to add two alignment constraints to place the views in the centre of the screen.
If you want to, you can place the two views inside a UIStackView to make positioning easier. To do this select the label and button, click the Embed In button in the bottom right of the screen (fifth from the left) and choose Stack View. You can modify how the stack view positions its containing views.
To align this stack view, select it and click the Align button in the bottom right of the screen (second from the left) and add Horizontally in Container and Vertically in Container constraints. This should position the label and button in the centre of the device.
If you don't want the label and button in a UIStackView you can always add constraints relative to each other.
Hope this helps.