That app does not allow timer on products integrated on the front page. Their website says:
"Our app is designed to insert the inventory/scarcity timer into the product page template.
However, products that are integrated into the front page do not use the product page template, and are not detected by the app. We are working on a feature to make this detection possible - please stay tuned for future updates!"
Another option is to edit your front page template yourself and incorporate the code for displaying the timer into it manually.