Enable only on Checkout

Yeah ?? set the display type to “shortcode” in the settings. You can then paste the shortcode on the checkout page, or use a function to place the shortcode.

Have a look here for an example of doing it through the functions.php

https://extrawoo.com/docs/checkout-countdown-for-woocommerce/how-to/place-shortcode-templates-or-hooks/

Yes, but that places it after the header, not before as your plugin does ??