Navigate to the Theme Settings > Shop > Promo popup and make sure it has been enabled.
Next, navigate to Appearance > Menu > check the menu item and make sure you have inserted woodmart-open-newsletter into CSS Classes (optional) http://prntscr.com/kysums
Have you created it with a plugin? Refer to the plugin documentation.
Besides the popup in the Theme Settings > Shop > Promo popup, you can create a popup by means of the WP Bakey Page builder element http://prntscr.com/kz2nh0