You did not add the Newsletter element into the header content, clear cache and check now. The demo “Newsletter popup” static block was created using Elementor, so can be edited using Elementor only. Go to Elementor > Settings > Post types > select Static Blocks, then go to Static blocks and edit the demo popup. Or select the desired popup static block, that you created from scratch using WPB or El (not both together), via Theme options > Header builder > Newsletter.
