Wait you *do* see the button? Then what happens when you press it?
As I said, the UI should be scaling fine, but it hasn't been tested above 2k or outside 16:9 so I theorized the button might have gone off screen. (Obviously its scaling with screen size, but the anchor might have been wrong). But if you both can place towers, and spawn the wave of enemies, then I don't quite get what you mean by "but nothing"...