If you are using the old version(not the store 2.0) theme. You need to install the popup code snippet code in your theme.
- Copy the popup snippet code in the Popup & Bubble section in the form design page.
- From your Shopify admin, go to Online Store > Themes.
- Find the theme that you want to edit, and then click Edit code.
- Click to open Layout/theme.liquid. Paste the popup snippet code included in the </head> tag, Save the file.
Operation Demo: