How preview Main.storyboard in different iphone/ipad screen sizes?

I'd like to look how my layout is on a different iphone screen sizes.

There are alot of tutorials but it doesn't work for me.

What I've tried:

• Opened Main.storyboard and opened the assistent editor


• In the assistent editor I chose "Preview" -> "Main.storyboard(Preview)"


• Now it should be possible to add different screen sizes by clicking on the add button on the bottom left of the assistent editor but the add button is missing

What have I done wrong?