This happens when you install a new theme after app is installed. You would need to take following steps to add section to your homepage.
1. Visit your stores admin panel.
2. Click on "Online Store" > "Themes" > "Customize".
3. From left side panel, click on "Add section".
4. Find and click on "Custom HTML" and then click "Add".
5. Remove existing code from text area and copy paste following code in there.
6. Now you can drag and move newly added section (Custom HTML) to appropriate section of homepage.
7. Enable app and you should see section on your homepage.