Great question! I definitely understand wanting to create a sense of urgency for customers so that they are entice to take advantage of any deals or promotions you might be running in your store.
@Uhrick is correct, you can only do this if you have access to checkout extensibility, which is available to our Plus merchants (high GMV stores). This is to help maintain the security of the checkout experience for all our merchants and their customers.
As you may have already discovered, there are lots of options to enable a countdown timer on your other pages in your store. It is important to note that countdown timers must be connected to actual promotional offerings or discounts in your store. A countdown timer designed to create a false sense of urgency is against our merchant TOS and can go against consumer protection laws in various countries. You can learn more about this here: Shopify Help Center | Consumer protection laws.
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