Some RuffRuff Selling Periods features are unavailable depending on the theme. First check whether the theme in use is Online Store 2.0.
On vintage themes, only the countdown timer is unavailable. The countdown timer is shown with an app block, so an Online Store 2.0 theme is required.
Cart button control and product messages through the app embed are available.
This app cannot be used on Baseline themes. The cart button implementation is not supported.