Check the code matches a coupon under Marketing → Coupons exactly. If no coupon has that code, the Cart Timer screen and the dashboard both say so in a warning box, and they also say so when coupons are switched off in WooCommerce, under WooCommerce → Settings → General → Enable the use of coupon codes. If a countdown campaign links the code, the Cart Timer screen says so too: WooCommerce then refuses it outside that campaign's time, so the cart timer can apply it only while the campaign runs. A coupon with its own rules — a minimum spend, say — is only applied once the cart meets them, and is applied by itself as soon as it does.
The timer's coupon also waits while the shopper has their own Individual use only coupon on the cart, rather than pushing it off or failing the checkout, and a timer coupon that is itself individual use is never applied over codes the shopper entered. Shoppers signed in while Show to is Guests only get no discount either.
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